lb-agent: fix get_interfaces_on_bridge returning None

Change get_interfaces_on_bridge() to return [] instead of None
if the bridge is not found, as callers are expecting an iterator.

Change-Id: I359a2a0cf932c049f6891d4ba3eb8b9b23ab360d
Related-Bug: 1284311
NSX plugin: fix get_gateway_devices

Fixes the error and adds relevant unit tests.
Also fixes get_gateway_devices signature for allowing
paging and sorting.

Closes-Bug: 1306301

Change-Id: Icb52a764e111365beaf4dc4fb4a5b8970a6cf887
Merge "lb-agent: fix get_interfaces_on_bridge returning None"
Merge "NSX plugin: fix get_gateway_devices"