tasks: generalize config writing for Filesystem

Signed-off-by: John Spray <john.spray@redhat.com>
tasks: generalise CephFSTestCase

Some of this stuff could be even more general for embedding
unittest-style suites, but for the moment let's keep the cephfs
stuff in a walled garden.

Signed-off-by: John Spray <john.spray@redhat.com>
tasks: add mds_client_limits

New CephFS tests for the behaviour of the system while
enforcing its resource limits.

Signed-off-by: John Spray <john.spray@redhat.com>
tasks: rename FuseMount.get_client_id to get_global_id

'client_id' was ambiguous because in other places it
meant the '0' in client.0, whereas here it means
the runtime-generated global ID of the client.

Signed-off-by: John Spray <john.spray@redhat.com>
tasks: wait for mds active before mounting clients

To make the logs clearer when trying to work out
if/when something went wrong, rather than always
having client logs start with some failures.

Signed-off-by: John Spray <john.spray@redhat.com>
suites: enable mds_client_limits test

Requires same cluster etc as 'recovery', though
we could now do with better name than that.

Signed-off-by: John Spray <john.spray@redhat.com>