Keystone: Move to SQL backend for Token storage

Using Memcache as storage backend for Tokens in Keystone, we hit this
bug: https://bugs.launchpad.net/keystone/+bug/1242620

It generates some bugs when running Tempest framework and also failures
in our QA process.

After some investigation, it seems the bug will not be fixed in Havana
and we should switch to SQL backend. Another work will be done
also on the MySQL keystone tables to avoid useless replication for
large scale deployments.

Bug #379
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Keystone / Rspec: Ensure we have SQL backend for tokens

Ensure in the tests that puppet-keystone provide us by default SQL
backend to store tokens.

Bug #379
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Keystone: Add token_driver param in cloud::identity

Allow the end-user to use another backend (memcache) but let SQL by
default.

Bug #379
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #392 from enovance/bug/379/emilien

Keystone: Move to SQL backend for Token storage