Handle os_auth_token option in keystone config

I88c2f5aa914b5fb40b6dc16a256d3e385d9aaa23 changed the option name to
auth_token for no good reason, breaking compatibility with current code.
Let's support both values.

Change-Id: I2b600b029a0e74265375dec1fe66651ab208eacd

zaqarclient/auth/keystone.py

@@ -65,17 +65,16 @@ class KeystoneAuth(base.AuthBackend):
             the auth information.
         """
 
-        token = self.conf.get('auth_token')
+        def get_options(k):
+            return self.conf.get(k, self.conf.get("os_%s" % k))
+
+        token = get_options('auth_token')
         if not token or not request.endpoint:
             # NOTE(flaper87): Lets assume all the
             # required information was provided
             # either through env variables or CLI
             # params. Let keystoneclient fail otherwise.
 
-            def get_options(k):
-                k = k if k in self.conf else "os_"+k
-                return self.conf.get(k, None)
-
             ks_kwargs = {}
             keys = ("username", "password", "project_id",
                     "project_name", "auth_url", "insecure",