Change behaviour to align with vanilla Keystone

The current code enforces that the tenant name is the same as the VO
name. This code removes this constrain, so that the user should use the
tenant discovery to get his tenant in advance.
Move code to keystone_voms.core and update docs

The code has been moved to keystone_voms/core.py so that it is not
anymore in __init__.py

The documentation has been updated to reflect the latest changes.
Update tests to work with Grizzly.

The unittests can now be executed against a Grizzly environment. Added
instructions to run them.
Merge pull request #4 from alvarolopez/remove_tenant_constraints

Remove tenant constraints