mgr/cephadm: create thread to start event loop for ssh.py, and return results of...

mgr/cephadm: create thread to start event loop for ssh.py, and return results of the async functions with get_result

The EventLoopThread class starts a thread and an event loop which runs forever. Coroutines are scheduled on the event loop by the `get_result` method which uses `run_coroutine_threadsafe` to return a concurrent.futures.Future, and ultimately the result with .result()

Fixes: https://tracker.ceph.com/issues/44676
Signed-off-by: Melissa Li <li.melissa.kun@gmail.com>