rolling_update: fix active mds host value

rolling_update: fix active mds host value

The active mds host should be based on the inventory hostname and not on
the ansible hostname.
The value returns under the mdsmap structure is based on the OS hostname
so we need to find the right node in the inventory with this value when
doing operation on inventory nodes.

Othewise we could see error like:

The task includes an option with an undefined variable. The error was:
"hostvars[foobar]" is undefined

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>