Disallow or discourage selection of networks which are not available to all hosts within the selected compute resource

[zjs]

Selecting a network which is not available to all hosts within a compute resource is undesirable. We should make sure users understand what they're doing if they select such a network.

[zjs]

This should not be fixed for 1.3.0.

@stuclem: We should ensure that this is documented in an appropriate fashion. We should make it clear to users that they should generally avoid selecting networks available to a subset of a selected cluster. We should consider a KB saying something like "if all of your container VMs are winding up on the same host, check your networking configuration". (If a release note is not the right way to document this, just remove kind/note.) See also: #72

[stuclem]

Created https://github.com/vmware/vic-product/issues/1189 for doc update. Removing kind/note.