rest-api.py: fix up client identification

The code to extract which clients to start the API on was confused;
fix it up.  Also, strip off 'client.' instead of 'clients' (since it
was done with len(), the result was the same, but confusing)

Signed-off-by: Dan Mick <dan.mick@inktank.com>
rest-api.py: test that ceph-rest-api is up and servicing requests

Wait for the daemon to actually be ready before continuing with other
tasks.  Loop 12 times, waiting 5s, before deciding something's wrong.

Fixes: #6243
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #120 from ceph/wip-6243

Wip 6243