Per-branch-plan capability requirements

Possibly covered by, or at least related to BAM-12232, we've begun to encounter situations in which branch plans may differ in their capability requirements: specifically, the situation I'm thinking of is where a release branch has been built using, say, JDK 1.6, but we're moving the techstack from 1.6 to 1.7; in this case, we might want to move most of the agents to the 1.7 capability, but keep an agent that only advertises 1.6 capabilities to ensure that hotfixes can continue to be built until we're fully able to confirm the ability of an agent to switch between 1.6 and 1.7.

This would apply to testing that requires specific environments, etc. The ability to specify a per-branch-plan capability requirement would require more setup on our end, but would guarantee the stability of branch builds in specific environments without having to promote a branch-plan to the top level to specify its available agents.