Add pythonic configuration

Update the configuration loading process and usage to be less like shell
environment variables and more like a typical python application.

* ConfigParser is used to read configuration from files.
* Common command line argument processing has been added to allow
  specification of a custom configuration file and/or overriding the
  values of individual configuration options.
* Config values are resolved in such a way as to allow configuration for
  multiple hosts and deployment environments to be specified in a single
  file.
* Configuration provided in /srv/scap/scap.cfg can be overridden at each
  host with an /etc/scap.cfg configuration file.
* All configuration values are returned as strings and must be type cast
  by the consumer.
* Settings have been renamed to provide more semantic meaning for users
  (eg 'stage_dir' instead of 'MW_COMMON_SOURCE').

Change-Id: Id8c7dc5d84c1e7a1e24143201e580930b0de8acc