cephadm: refactor call() using asyncio.asyncio.StreamReader

cephadm: refactor call() using asyncio.asyncio.StreamReader

simpler this way, also fix a couple issues:

* create a child watcher explicitly, see
  https://bugs.python.org/issue35621
* use StringIO for collecting outputs for better performance,
  instead of appending the lines to an existing str
* catch ValueError when reading from the stream reader,
  because StreamReader.readline() could raise ValueError when
  it reaches the buffer limit while looking for a separator.
  in this case, we should try again, in hope that the spawned
  process can feed the reader with more data which contains
  the separator (i.e., b'\n').
* backport ThreadedChildWatcher from Python 3.8 so we can
  run create_subprocess_exec() in non-main threads.

Signed-off-by: Kefu Chai <kchai@redhat.com>