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 <dan.mick@inktank.com>