Implement a proxy checker class.

Add a proxy checker class for local checks that doesn't do anything,
but instead registers a command option that allows for local, in-tree
checks.

Change-Id: I3f7567639ae47cc422c84f319f1b607ad193d901
Reviewed-on: https://review.openstack.org/28220
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Giampaolo Lauria <lauria@us.ibm.com>
Reviewed-by: Sean Dague <sean@dague.net>
Approved: Joe Gordon <jogo@cloudscaling.com>
Reviewed-by: Joe Gordon <jogo@cloudscaling.com>
Tested-by: Jenkins
Update indentation in setup.cfg.

Change-Id: I79d230aa834e58c482fbfa8d72cc1c4abee98fdc
Reviewed-on: https://review.openstack.org/28623
Reviewed-by: Joe Gordon <jogo@cloudscaling.com>
Approved: Sean Dague <sean@dague.net>
Reviewed-by: Sean Dague <sean@dague.net>
Tested-by: Jenkins