enhancement(is_region): made name as optional in ibm_is_region datasource

[ujjwal-ibm]

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Relates OR Closes #1258

Output from acceptance testing:

--- PASS: TestAccIBMISRegionDataSource_basic (35.59s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm 40.186s

[ujjwal-ibm]

rebased

=== RUN   TestAccIBMISRegionDataSource_basic
--- PASS: TestAccIBMISRegionDataSource_basic (29.89s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     31.770s

[ujjwal-ibm]

rebased

--- PASS: TestAccIBMISRegionDataSource_basic (22.70s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     24.871s