ganeti-watcher should attempt to fix ganeti-rapi

Update ganeti-watcher so that it tests the master's RAPI port with a
simple test (in this case GetVersion). If it fails, make one attempt
at restarting ganeti-rapi and retest.

- daemons/ganeti-watcher: Test rapi and make one attempt at restarting it.
- lib/utils.py: add StopDaemon() function.

Signed-off-by: Tom Limoncelli <tlim@google.com>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>