Handle "null" time values for Stacks

The timesince filter expects a date or datetime object, and would fail
when receiving an empty string. Create a new filter that returns the
string value "Never" under these circumstances.

Change-Id: I73f4dbb608fc143c3ac60d753e9e222762579e51
Closes-Bug: #1286959
Added network's api mocking for the servers_update_addresses method

Change-Id: Idd6ec125ac4c256c1edae445a7bc6a3a70ba4962
Closes-Bug: #1296321
Merge "Handle "null" time values for Stacks"
Merge "Added network's api mocking for the servers_update_addresses method"