Add a get_groups method that returns all the groups of interest in the database

These groups are the one used to restrict an election to a group of legal
voters or to specify a group as admin for an election.

We are interested in this list to ask OpenID to only return the groups
of interest rather than asking for all of them.
When login in, ask for the groups used in the database and admin groups set in the configuration
Merge pull request #32 from fedora-infra/restrict_groups

Restrict groups