govc: add cluster.mv command

[dougm]

Description

Move hosts into a cluster.

vcsim: add ClusterComputeResource.MoveInto_Task support

Closes #2524

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 new bats tests

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

[hickeng]

matching test for moving a host out of a cluster to become standalone?

[dougm]

cluster.mv (MoveInto_Task) can only move hosts into a cluster (from standalone or another cluster).
object.mv (MoveIntoFolder_Task) can move from a cluster into a host folder and it becomes a standalone host (just learned that tonight). Let's revisit that case, at the moment vcsim only supports moving from Folders:

govmomi/simulator/folder.go

Lines 567 to 571 in c715016

	parent, ok := ctx.Map.Get(*(obj.Entity()).Parent).(*Folder)
	if !ok || !folderHasChildType(&f.Folder, ref.Type) {
	return nil, &types.NotSupported{}
	}