NSX: Do a single query for all gateway devices

When creating a network gateway, do a single query for all
gateway devices, rather than a distinct query for each device.

Change-Id: I9e66539eb36ae75d3dcdd8a9073d2a2f69447e98
Closes-Bug: 1320278
Make pid file locking non-blocking

fcntl.flock will block indefinitely if another process holds an exclusive
lock. A non-blocking flock operation will raise an error when a lock already
exists so we can fail immediately.

Closes-bug: 1315507
Change-Id: Icf97b1f8643157719b3d28ac2c0c1576a5069697
Merge "NSX: Do a single query for all gateway devices"