engine: Reinitialize VDSManager on change cluster

When changing cluster, the VDSManager dictionary
has to be updated.
This is needed because the target cluster may support
different services from the source cluster, and the
monitoring strategies need to be updated.

Change-Id: Icb25c12d35113af09c67d5fe6e275c6901725e54
Bug-Url: https://bugzilla.redhat.com/968178
Signed-off-by: Sahina Bose <sabose@redhat.com>