Add support for externally managed VCN

[shyamradhakrishnan]

What this PR does / why we need it:
Support use case where VCN can be managed externally but API Server LB can be managed by CAPOCI.

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 #64

[shyamradhakrishnan]

Unit test

setting up env vars
?   	github.com/oracle/cluster-api-provider-oci	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/api/v1beta1	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/config	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/cloud/ociutil	0.611s	coverage: 30.3% of statements
ok  	github.com/oracle/cluster-api-provider-oci/cloud/scope	181.262s	coverage: 81.9% 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/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	28.586s	coverage: 70.7% of statements

Externally managed e2e test ran fine

• [SLOW TEST:545.503 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 1 of 21 Specs in 672.995 seconds
SUCCESS! -- 1 Passed | 0 Failed | 0 Pending | 20 Skipped

Only CCM test failed, because of an infra issue in env, public IP limit was reached

Summarizing 1 Failure:

[Fail] Workload cluster creation [It] Cloud Provider OCI testing [PRBlocking]
/home/ubuntu/go/src/github.com/oracle/cluster-api-provider-oci/test/e2e/ccm_helpers.go:83

Ran 5 of 21 Specs in 1707.570 seconds
FAIL! -- 4 Passed | 1 Failed | 0 Pending | 16 Skipped