rbd-wnbd: use one daemon process per host

rbd-wnbd: use one daemon process per host

We're currently using one rbd-wnbd process per image mapping.
Since OSD connections aren't shared across those processes,
we end up with an excessive amount of TCP sessions, potentially
exceeding Windows limits:
https://ask.cloudbase.it/question/3598/ceph-for-windows-tcp-session-count/

In order to improve rbd-wnbd's scalability, we're going to use
a single process per host (unless "-f" is passed when mapping the
image, in which case the daemon will run as part of the same
process). This allows OSD sessions to be shared across image
mappings.

Another advantage is that the "ceph-rbd" service starts faster,
especially when having a large number of image mappings.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>