Make dimensions configurable

 - Add dimensions to sample configuration
 - Define and load dimensions dict in Checkable.py
 - [cv] section required in test config for Checkable and Dimension tests
 - Improve message when no configuration is found, with test.

Change-Id: Ia3aff8b3ca453f043dbba6a4b4ae49e41b78d581
Moving DBI calls to library files, test tweaks

 - Move DBI call for interactive drop table down into crawl_lib.py
 - Move DBI calls for checksum, match, failure counts from cv_plugin.py into
   cv_lib.py
 - Added file messages.py where we'll put common messages used in multiple
   files
 - Added util.squash() which takes a string and returns a version of it with
   internal whitespace runs squeezed down to a single space and leading and
   trailing whitespace stripped off
 - There are still more DBI calls in plugins and tools to be moved to the
   library files

Change-Id: Icdeeb5fc9c7983fab5ef377b936a301dabf86fc8