Cells: Add cells commands to nova-manage

Add commands to nova-manage for configuring cells in the DB.

Implements blueprint nova-compute-cells

DocImpact

Change-Id: Ic1a950181d5ce191780647838c5d06e5410676f7
Add helper methods to nova.paths

We only ever do one of two things with the global path options:

  1) Reference the option in the default of another option so that the
     value can be interpolated it

  2) Use the value of the option to build a path

Add helper methods for both these cases - e.g. basedir_def() for the
former case and basedir_rel() for the latter case. This makes it much
more obvious how and where these options are used.

Change-Id: I7fd94a329fe911761d02d94e5381e950c6668d56
Merge "Add helper methods to nova.paths"
Merge "Cells: Add cells commands to nova-manage"