Feature: Allow matching group RDN (Relative Distinguished Name) in group_dns opt

[cognifloyd]

Adds support for RDN matching of group names. For example:

[auth]
mode = standalone
backend = ldap
backend_kwargs = {
                   ...
                   "group_dns": [
                     "CN=st2users", # matches "CN=st2users,ou=groups,dc=example,dc=com",
                     "CN=st2developers" # matches "CN=st2developers,ou=groups,dc=example,dc=com"
                   ],
                 }

Closes #104

Depends on #116