bkr workflow commands: Predefined host filters

This patch adds the capability to make use of predefined host filters
with the bkr workflow commands. A default list of filters will be
installed when beaker-client is installed and can be supplemented with
the users' own set of filters.

Bug: 1081390
Change-Id: I58c3d61623ebfce02a51ba3ff1333557f60850c1