task: Add mds_creation_failure

This is test code to accompany CephFS fix #7485.

Also fix DaemonState.wait_for_exit to clear up its 'proc'
attribute even if it fails, so that subsequent calls to 'restart'
happen properly.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #208 from ceph/wip-7485

task: Add mds_creation_failure

Reviewed-by: Greg Farnum <greg@inktank.com>