Add __init__.py for nova scheduler filters

To use these filters out of tree right now, you have to specify them in
nova's available_scheduler filters by full path. But in order for the
modules to be found and loaded, you must have an __init__.py.

Closes-Bug: #1357878
Change-Id: I4816d9dbe0e93056792fad48e6228ffdd531e346
(cherry picked from commit 87aac8823b468a7bb80c994cd207ec5df66163c5)
Merge "Add __init__.py for nova scheduler filters" into stable/icehouse