resource/aws_db_instance: Prevent double apply for snapshot_identifier with multiple other arguments

[bflad]

Closes #2869
Fixes #2859
Reference (this is the first half of these issues -- pending-reboot issue to be fixed after): #218, #1494, #1510

Previously:

--- FAIL: TestAccAWSDBInstance_SnapshotIdentifier_BackupRetentionPeriod (1154.50s)
	testing.go:527: Step 0 error: Check failed: Check 4/4 error: aws_db_instance.test: Attribute 'backup_retention_period' expected "1", got "0"
--- FAIL: TestAccAWSDBInstance_SnapshotIdentifier_BackupWindow (1195.82s)
	testing.go:527: Step 0 error: Check failed: Check 4/4 error: aws_db_instance.test: Attribute 'backup_window' expected "00:00-08:00", got "06:13-06:43"
--- FAIL: TestAccAWSDBInstance_SnapshotIdentifier_IamDatabaseAuthenticationEnabled (1113.22s)
	testing.go:527: Step 0 error: Check failed: Check 4/4 error: aws_db_instance.test: Attribute 'iam_database_authentication_enabled' expected "true", got "false"
--- FAIL: TestAccAWSDBInstance_SnapshotIdentifier_MaintenanceWindow (1259.26s)
	testing.go:527: Step 0 error: Check failed: Check 4/4 error: aws_db_instance.test: Attribute 'maintenance_window' expected "Sun:01:00-Sun:01:30", got "wed:10:33-wed:11:03"
--- FAIL: TestAccAWSDBInstance_SnapshotIdentifier_Monitoring (1214.60s)
	testing.go:527: Step 0 error: Check failed: Check 4/4 error: aws_db_instance.test: Attribute 'monitoring_interval' expected "5", got "0"
--- FAIL: TestAccAWSDBInstance_SnapshotIdentifier_ParameterGroupName (1134.54s)
	testing.go:527: Step 0 error: Check failed: Check 4/4 error: aws_db_instance.test: Attribute 'parameter_group_name' expected "tf-acc-test-4546585708384390733", got "default.mariadb10.2"

Changes proposed in this pull request:

• Ensure the following Terraform arguments trigger ModifyDBInstance when using RestoreDBInstanceFromDBSnapshot
  • backup_retention_period
  • backup_window
  • iam_database_authentication_enabled
  • maintenance_window
  • monitoring_interval
  • monitoring_role_arn
  • parameter_group_name

Output from acceptance testing:

34 tests passed (all tests)
--- PASS: TestAccAWSDBInstance_importBasic (441.25s)
--- PASS: TestAccAWSDBInstance_iamAuth (451.00s)
--- PASS: TestAccAWSDBInstance_generatedName (471.69s)
--- PASS: TestAccAWSDBInstance_basic (511.88s)
--- PASS: TestAccAWSDBInstance_namePrefix (512.04s)
--- PASS: TestAccAWSDBInstance_kmsKey (520.80s)
--- PASS: TestAccAWSDBInstance_optionGroup (542.32s)
--- PASS: TestAccAWSDBInstance_noSnapshot (663.43s)
--- PASS: TestAccAWSDBInstance_s3 (837.33s)
--- PASS: TestAccAWSDBInstance_subnetGroup (905.89s)
--- PASS: TestAccAWSDBInstance_snapshot (986.80s)
--- PASS: TestAccAWSDBInstance_portUpdate (552.60s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier (1159.29s)
--- PASS: TestAccAWSDBInstance_separate_iops_update (664.15s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_MaintenanceWindow (1209.77s)
--- PASS: TestAccAWSDBInstance_MinorVersion (449.18s)
--- PASS: TestAccAWSDBInstance_enhancedMonitoring (777.38s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_IamDatabaseAuthenticationEnabled (1329.91s)
--- PASS: TestAccAWSDBInstance_diffSuppressInitialState (448.94s)
--- PASS: TestAccAWSDBInstance_replica (1372.02s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_BackupWindow (1491.56s)
--- PASS: TestAccAWSDBInstance_ec2Classic (523.50s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_Monitoring (1524.52s)
--- PASS: TestAccAWSDBInstance_cloudwatchLogsExportConfiguration (453.37s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_BackupRetentionPeriod (1664.06s)
--- PASS: TestAccAWSDBInstance_cloudwatchLogsExportConfigurationUpdate (577.94s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_ParameterGroupName (1309.21s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_Tags (1348.55s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_VpcSecurityGroupIds (1350.06s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_VpcSecurityGroupIds_Tags (1380.31s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_MultiAZ (1936.30s)
--- PASS: TestAccAWSDBInstance_EnabledCloudwatchLogsExports_Oracle (782.88s)
--- PASS: TestAccAWSDBInstance_MSSQL_TZ (3030.58s)
--- PASS: TestAccAWSDBInstance_SnapshotIdentifier_MultiAZ_SQLServer (3984.31s)

[mbfrahry]

LGTM

[ghost]

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