Packages: Added timeout to bcfg2-yum-helper calls

This involved making the Yum backend use Bcfg2.Utils.Executor to call
bcfg2-yum-helper instead of subprocess.Popen directly.  This was
cherry-picked (kinda) from 3d06f311274d6b942ee89d8cdb13b2ecc99af1b0,
so will likely break the maint -> master merge in spectacular ways.