admin_socket: loop until the socket command succeeds

Sometimes the thing we're talking to is slow to start, or to register the
command we are running.  Loop in that case, at least for a while.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 222b296019d8d88d3d2fd3def947658bc076dcac)

Conflicts:
	teuthology/task/admin_socket.py