Use standard ConfigParser for updating configs.

Use python standard ConfigParser for updating configs now that all
OS projects have standardized on a config format.  Replaces the
use of sed, which is hard to support across versions.  Use the same
set_or_update() to set keystone paste_deploy flavor now.

Drops a couple of unused functions.
merging lp:~gandelman-a/charms/precise/glance/cfgparser