engine: Avoid race condition between gluster peer probe and status

During add of a gluster host, a gluster peer probe command is executed.
If a gluster peer status is also fired at the same time from GlusterSyncJob
this results in the newly added host being removed from the engine database,
as the gluster peer status has not yet recognized the newly added host.

Added lock at cluster level before peer probe to avoid this.

Change-Id: Iaa76cea4c5b5bf7c444680260dd4da5b65665428
Bug-Url: https://bugzilla.redhat.com/972619
Signed-off-by: Sahina Bose <sabose@redhat.com>