engine: Lock add network method

Add network command should be locked on network name to prevent a case
when multiple networks with the same name are being added.

Change-Id: Ie7b366432cf946eb5ec8ab1ffd36a97837ff226a
Bug-Url: https://bugzilla.redhat.com/990100
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>