Improve Content Library test

[adambarreiro]

Before this PR, to run Content Library test one required to set SubscriptionContentLibraryUrl, which forced to create a publisher library in vCenter just for this test.

Now, the Content Library tests create the publisher library on-the-fly by leveraging ExternalProviders property and the vSphere Terraform Provider. This way we don't need the URL anymore. In exchange, we now require a vCenter Datacenter and Datastore, which are trivial to obtain and don't change frequently, also they don't need to be created for the test.

• TestAccVcfaContentLibrary* acceptance tests pass

go test -tags functional -vcfa-skip-priority-tests -vcfa-add-provider -run '^TestAccVcfaContentLibrary(Tenant|Provider)$' -v -timeout 0

• vcfa.TestAccVcfaContentLibrary*.tf tests pass:

make test-binary-prepare
cd vcfa/test-artifacts
./test-binary.sh names 'vcfa.TestAccVcfaContentLibraryProvider*.tf' 
./test-binary.sh names 'vcfa.TestAccVcfaContentLibraryTenant*.tf' 

[lvirbalas]

Very nice!