Export token api

Expose the 'GET /token' api entry point and a couple of OpenStack's clients factories (nova, neutron, swift). This allows customer to use OpenStack's API without putting admin passwords in their applications. The factories are a real added value as it is extremely tricky to figure the right calling convention as there is literally no documentation on the clients. These factories use lazy imports so that `python-neutronclient` and `python-novaclient` are not strong requirements.