Specify nova configuration file with --config-dir

This change use '--config-dir' option to indicate specific configuration
folder to store configuration files for setting nova services.
That allows, for exmaple, to define the same setting with different
value for two different nova services.

Change-Id: I77c382f05128112102959e1370736fc8564bcb1f
Closes-bug: #1326016
Merge "Specify nova configuration file with --config-dir"