intg: Make location of sssd nss module configurable
    
    The path to sssd nss module (libsss_nss.so) was
    relative to prefix and expected subdirectory "lib".
    32bit and 64bit platforms and different distributions
    use different paths. This patch allows to use python module sssd_id
    even with real module and not just integration tests.
    It is just required to prepare "config.py" with right path.
    
    e.g.
      cd ~/sssd/src/tests/intg
      [~/sssd/src/tests/intg]$ echo "NSS_MODULE_DIR = '/usr/lib64'" > config.py
      [~/sssd/src/tests/intg]$ python
      Python 2.7.12 (default, Jul 18 2016, 09:57:01)
      [GCC 6.1.1 20160621 (Red Hat 6.1.1-3)] on linux2
      Type "help", "copyright", "credits" or "license" for more information.
      >>> import sssd_id
      >>> sssd_id.get_user_gids('user')
      (1, 0, [5977, 1070, 5845, 1076, 1074, 10327, 5975, 5766])
    
    Reviewed-by: Petr Čech <pcech@redhat.com>
    (cherry picked from commit 70d47ad4c89152f1e2ac4599f75f9374b631b953)