Fix CellStateManagerFile init to failure

Currently, specifying a cells_config file in nova.conf causes
CellStateManager to fail and in turn stops the nova-cells service from
starting. The reason is that CellsManager creates an instance of
CellStateManager with no arguments. CellStateManager __new__ runs and
creates an instance of CellStateManagerFile which runs __new__ and
__init__ with cell_state_cls and cells_config_path set. At this point
__new__ returns CellStateManagerFile and the new instance's __init__
method is invoked (CellStateManagerFile.__init__) with the original
arguments (there weren't any) which then results in:
2014-04-29 11:52:05.240 16759 TRACE nova self.state_manager =
cell_state_manager()
2014-04-29 11:52:05.240 16759 TRACE nova TypeError: __init__() takes
exactly 3 arguments (1 given)

It seems reasonable for CellStateManagerFile to derive the
cells_config_path info for itself so I have updated the code with that
change and added unit tests to catch this bug and to check that the
correct managers are still returned

Change-Id: I9021640515142a3ca95c2d9e7b03e19b529bc175
Closes-Bug: #1314677
Merge "Fix CellStateManagerFile init to failure"