Allocate unique key in etcd using "create"
Allocate unique key in etcd using "create"

Create use atomic transaction in order to verify that
the value wasn't exist before updating the DB.

This should solve our race-condition. If two (or more)
allocations will occure at the same time, after the first
allocation will succeed, the others will fail.

Change-Id: Id72c7c35415a68c1eebc05ed3126f59793d5c475
Closes-bug: #1713496

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow