Title: #390: NSS: Add option to disable memcache
@lslebodn, your request is out of the scope of this patch.
It is not because if we decide that we do not want to document discouraged options by
this PR need to be changed. BTW discouraged means also that they are not documented and
they are is not tested by upstream.
See the full comment at https://github.com/SSSD/sssd/pull/390#issuecomment-331878670