Provisioning should support Beaker pool enforcement/exclusion

[happz]

In Beaker jobs, this is possible to request/exclude a specific Beaker pool (not to be confused with Artemis pools):

system-pool: !virtkvm-gating
system-pool: virtkvm-gating

First, it's a feature sought by current Beaker users, and it probably shouldn't be hard to extend the Beaker plugin/mrack to support this.

Second, I'm not sure how to support this with Artemis, and to what extent this should be decoupled from how: beaker. As Artemis can and does wrap Beaker service too, it can certainly accept constraints of this kind and propagate them to Beaker behind the curtain. Feels like something generic enough for hardware, but it's not actual HW requirements, more like an organizational aspect... Anyway, adding HW labels, open for discussion.

[thrix]

Relevant RH issues:
https://issues.redhat.com/browse/KQE-59