Force selected variables to be refreshed when build is rerun or restarted

By default, Bamboo creates snapshot of global and plan variables when starting a build. This is done in order to make sure that build environment does not change between restarts. This is not, however, a desirable behavior in all cases, the credentials being one example.

As of Bamboo 7.0, this no longer applies to 'password' variables. Those values are taken straight from their definition.

Proposal: make it possible to explicitly mark variables as 'transient' (or 'volatile': exact wording TBD), that is, variables that are not a part of the snapshot and need to be always taken straight from the definition.