Fixes bug 1088801
devstack does not create signing_dir when keystone token format is UUID.
If the default value of signing_dir is read-only, OpenStack services
such as Quantum server failed to start due to permission denied.
On the keystone client cannot know which token_format is used in keystone
in advance, so signing_dir should be created regardless of the token format.
Change-Id: I1b0d25c1ac4d22d9fb2c5443d15b96fdaa5a4c81