Add meta-config via local.conf

This defines a new local.conf file that is designed to take the place of all
of the 'pass-through'[1] configuration options that have been defined in DevStack.

* new local.conf file can contain multiple config file settings to be
  merged in to existing project config files
* localrc can be embedded into local.conf and will auto-extract if
  localrc does not exist
* Adds functions get_meta_section_files(), get_meta_section(),
  merge_config_file() and merge_config_group()

[1] Pass-through options are those that do not configure or change DevStack's behaviour
but are simply set in a project config file.  This includes most of the EXTRA_XXX_OPTS
configuration variables.

Change-Id: I367cadc86116621e9574ac203aafdab483d810d3
Merge branch 'config-local' into dt-test