qa/suites/rbd: use client.0 entity in migration-external tests

qa/suites/rbd: use client.0 entity in migration-external tests

Currently client.admin is passed for client_name and that doesn't
exercise client_name handling much as client.admin is the default.

When deploying multiple clusters the ceph task distributes keyrings
with only the key for the initial monitor and client.admin key.  Other
keys (e.g. client.0) are present only on their respective clusters, so
client.0's key for cluster2 needs to be obtained on cluster1 explicitly
with "ceph auth get".

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>