nixos/keystone: init at liberty version

This commit introduces a nixos module for the Openstack Keystone
service. It also provides a optional bootstrap step that creates some
basic initial resources (tenants, endpoints,...).

The provided test starts Keystone by enabling bootstrapping and checks
if user creation works well.

This commit is based on initial works made by domenkozar.

nixos/modules/misc/ids.nix

@@ -280,6 +280,7 @@
       stanchion = 262;
       riak-cs = 263;
       infinoted = 264;
+      keystone = 265;
 
       # When adding a uid, make sure it doesn't match an existing gid. And don't use uids above 399!
 
@@ -530,6 +531,7 @@
       stanchion = 262;
       riak-cs = 263;
       infinoted = 264;
+      keystone = 265;
 
       # When adding a gid, make sure it doesn't match an existing
       # uid. Users and groups with the same name should have equal