Add APIs for vLCM enablement on a cluster and base ESXi image selection

[spacegospod]

Description

This change is a follow-up to #3368
I am adding a some new methods to the vLCM bindings which allow for

1. Enabling vCLM on a cluster
2. Selecting a base ESXi image for the cluster
3. Retrieving the list of available ESXi images

I also refactored the simulator for the vLCM endpoints

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Build related change

How Has This Been Tested?

Added more test cases to vlcm.bats

Checklist:

• My code follows the CONTRIBUTION guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[dougm]

lgtm, thanks @stoyanzhelyazkov