task: refactor kclient into KernelMount

Additionally make both kclient and ceph_fuse
tasks yield a collection of CephFSMount
objects so that subsequent tasks can
retrieve them.

Signed-off-by: John Spray <john.spray@redhat.com>
task/mds_journal_migration: only mount my client

Pass explicit client ID list to ceph_fuse context so
that it doesn't try to mount all clients in the config,
in case there are other clients.

Also allow configuration to specify which client
this task should use.

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

This task exercises the CephFS session recovery
behavior.

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