P4 source changes:

* Made `mode` parameter support property interpolation
* Added ticket-based authentication
Added support for ticket-based authentication to P4Poller.
Added ticket login interval to P4Poller.
Some code cleanup.
P4Poller: fix ticket-based authentication, add optional debug logging.
Default P4Poller's ticket fetching interval to 24 hours.
Updated documentation and release notes about support for Perforce ticket-based authentication.
Added unit test for ticket auth in P4 source step.
Add a unit test for ticket auth in P4 poller.
Merge ludovicchabant/buildbot:perforce (PR #1128)

Conflicts:
	master/buildbot/changes/p4poller.py
	master/buildbot/test/unit/test_changes_p4poller.py
	master/buildbot/test/unit/test_steps_source_p4.py
	master/docs/relnotes/index.rst

+ autopep8
+ substantial merging in tests