Sync periodic_task module from oslo-incubator

The periodic_task module hasn't been sync'd since the middle of the
Juno release cycle.  This patch brings it up to date with the
latest oslo-incubator code.

Current HEAD in OSLO:
---------------------
commit 36b0e8570b449129d6d474c03b02ceb62edb78df
Date:   Thu Dec 11 11:27:08 2014 +0100
We shouldn't replace `oslo-incubator` in comments

Changes merged with this patch:
---------------------
5d40e143 - Remove code that moved to oslo.i18n
a3220c51 - add list_opts to all modules with configuration options

Change-Id: I0d68ba54617be550244969b7672e95158126f64a
Merge "Sync periodic_task module from oslo-incubator"