Add doc for instance principal based e2e and fix empty subnet cidr bug

[shyamradhakrishnan]

What this PR does / why we need it:
The PR adds the following

• Doc to supports instance principal based e2e testing
• Minor changes for instance principal based e2e testing
• Fix the empty subnet name and CIDR bug, CAPOCI should allow empty values

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #83 , Fixes #47

[shyamradhakrishnan]

Unit test output

?   	github.com/oracle/cluster-api-provider-oci	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/api/v1beta1	1.160s	coverage: 18.7% of statements
?   	github.com/oracle/cluster-api-provider-oci/cloud/config	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/cloud/ociutil	0.532s	coverage: 25.0% of statements
ok  	github.com/oracle/cluster-api-provider-oci/cloud/scope	178.142s	coverage: 74.5% of statements
?   	github.com/oracle/cluster-api-provider-oci/cloud/scope/mocks	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/compute	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/compute/mock_compute	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/computemanagement	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/identity	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/identity/mock_identity	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/networkloadbalancer	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/networkloadbalancer/mock_nlb	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/vcn	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/vcn/mock_vcn	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/controllers	27.564s	coverage: 71.0% of statements
?   	github.com/oracle/cluster-api-provider-oci/exp/api/v1beta1	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/exp/controllers	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/feature	[no test files]

[shyamradhakrishnan]

e2e test output

• [SLOW TEST:494.752 seconds]
Workload cluster creation
/home/ubuntu/go/src/github.com/oracle/cluster-api-provider-oci/test/e2e/cluster_test.go:49
  Externally managed VCN
  /home/ubuntu/go/src/github.com/oracle/cluster-api-provider-oci/test/e2e/cluster_test.go:493
------------------------------
STEP: Tearing down the management cluster


Ran 14 of 21 Specs in 4043.517 seconds
SUCCESS! -- 14 Passed | 0 Failed | 0 Pending | 7 Skipped