Add documentation for worker node customization

[shyamradhakrishnan]

What this PR does / why we need it:

Add documentation for customising worker nodes and fix misc bugs

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 #161

[shyamradhakrishnan]

unit tests

Using cached envtest tools from /Users/shyaradh/oke/src/github.com/oracle/cluster-api-provider-oci/testbin
setting up env vars
?   	github.com/oracle/cluster-api-provider-oci	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/api/v1beta1	1.630s	coverage: 17.9% of statements
?   	github.com/oracle/cluster-api-provider-oci/cloud/config	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/cloud/ociutil	1.286s	coverage: 25.0% of statements
ok  	github.com/oracle/cluster-api-provider-oci/cloud/scope	168.656s	coverage: 73.2% of statements
?   	github.com/oracle/cluster-api-provider-oci/cloud/scope/mocks	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/base	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/base/mock_base	[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/containerengine	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/containerengine/mock_containerengine	[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	28.241s	coverage: 69.0% of statements
ok  	github.com/oracle/cluster-api-provider-oci/exp/api/v1beta1	2.303s	coverage: 10.1% of statements
ok  	github.com/oracle/cluster-api-provider-oci/exp/controllers	1.936s	coverage: 43.4% of statements
?   	github.com/oracle/cluster-api-provider-oci/feature	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/version	[no test files]