Add a bunch of LDAP tests.

    * Adding/removing a user/group/membership with rfc2307(bis) schema.
    * Filtering users/groups with rfc2307(bis) schema.
    * The effect of override_homedir option.
    * The effect of fallback_homedir option.
    * The effect of override_shell option.
    * The effect of shell_fallback option.
    * The effect of default_shell option.
    * The effect of vetoed_shells option.

Fix creation of mixed user/group "member" attribute for RFC2307bis
group entries in ldap_ent.py.

Use a function to generate basic sssd.conf in test_ldap.py to reduce
code duplication.

Split ldap_test.py fixtures into several functions to allow for partial
fixtures and direct use within tests.

Support passing all user attributes to ldap_ent.py's user-creation
functions, in integration tests.

Remove "_rfc2307" from integration test function names for brevity.

Don't use the global LDAP_BASE_DN in integration tests and fixtures, but
instead take it from the LDAP connection object (ldap_conn) passed to
them explicitly. This makes the tests and fixtures a bit more modular.

Memory context was not freed therefore we got stuck in tevent loop
that mocks D-Bus.

Resolves:
https://fedorahosted.org/sssd/ticket/2759

Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>