Add support for excluding/ignoring VMs by Custom Attribute

[atc0005]

Ran into a situation where it would be useful to filter VMs by Custom Attribute.

For example, out of a larger subset of VMs for a group only three are under the management of a sysadmin not part of that group. As the VMs are organized within a folder named by the owning team (in this example) we cannot filter by the folder name.

Once #809 is implemented we could (and likely will for the short term) filter by specific VM names, filtering by a Custom Attribute could offer a much more flexible solution that doesn't require constant upkeep to reflect changes in VM names (e.g., dropping a VM, adding another).

This could be used in the same way that excluding/including resource pools are currently supported.