Allow having variables in agent capabilities and job requirements

Allow having variables in the job requirements and remote agent capabilities so that by changing a value of one variable (which can possibly be used in many job requirements) a user can change the agents that satisfies the requirements.

Variables in job requirements could also allow to run / not run a job based on which branch is currently being built, probably by using the variable ${bamboo.repository.branch.name}

Workaround

The problem that is highlighted in this issue can be reduced to modifying effective configuration of multiple plans with a simple change in Bamboo configuration (editing global variable).
We believe that Bamboo allows for such behavior with use of Bamboo Specs. Moving configuration of Bamboo plans to code allows modifying configuration of multiple Plans at the same time and much more.