misc.py: helper roles_to_remotes for generic use
misc.py: helper roles_to_remotes for generic use

roles_to_remotes will take a config key, 'roles' by default,
and search cluster.remotes for those roles, returning a list
of remotes that hold it.  The list is uniquified by default,
but that can be overridden.  So you can have a task that
should run on specific roles, configure it with
"roles: [mon.0 server.0 osd.2]", and call this to get a list
of remotes that contain those roles.

Signed-off-by: Dan Mick <b07550071eaa6a9296289c43bbd6c90559196431@inktank.com>

michaelsevilla/teuthology,t-miyamae/teuthology,yghannam/teuthology,dreamhost/teuthology,dmick/teuthology,ktdreyer/teuthology,dreamhost/teuthology,dmick/teuthology,michaelsevilla/teuthology,robbat2/teuthology,ceph/teuthology,ktdreyer/teuthology,zhouyuan/teuthology,zhouyuan/teuthology,SUSE/teuthology,ivotron/teuthology,robbat2/teuthology,yghannam/teuthology,t-miyamae/teuthology,tchaikov/teuthology,dmick/teuthology,ceph/teuthology,tchaikov/teuthology,caibo2014/teuthology,SUSE/teuthology,ivotron/teuthology,SUSE/teuthology,caibo2014/teuthology