Add cells get_cell_type() method

This moves/consolidates some the deprecated config code for cells into
nova/cells/opts.py and creates a new method to determine the cell_type
that we can use in other places where logic needs to differ depending on
'compute' or 'api' cells.

This also may happen to fix bug 1197091.  I'm unable to reproduce that
bug in tests, unfortunately.

We'll need this soon for some object related conversion.

Related to blueprint unified-object-model

Change-Id: Id291759fabb71656be18fb1eb405fcd5983f84ae
Merge "Add cells get_cell_type() method"