Clear enabled apps cache after loading authentication app

Since getEnabledApps() depends on an authentication app to be loaded,
especially in the case of LDAP, the cache from getEnabledApps() is now
cleared to make sure that subsequent calls will properly return apps
that were enabled for groups.
This is because getEnabledApps() uses the inGroups() function from the
group manager provided by LDAP or any other authentication app.

Fixes https://github.com/owncloud/core/issues/11484

TODO
- [ ] add unit tests