Cache cleaning when subadmin adds user to group

This commit fix an error happening when the subadmin tries to create an
user, adding him/her to the group s/he is subadmin of, using a LDAP
User/Group plugin.

This just forces the cache to be reset after an user is added to a
group.

Signed-off-by: Vinicius Cubas Brand <viniciuscb@gmail.com>