Clean-up in ClientBase

* Refactored code
* Simplified find_mixin
Updated Keystone authN plugin, tenant disc
Fixed tenant retrieval in keystone authN plugin
Small fix, added TODO
Minor fixes, changes in dir structure
Fixed reaction to empty tenants array, keystone
Fixed authenticate for Keystone

* HEAD is not supported in OCCI-OS
Fixed fallback to default entity type
Fixed location in response to POST
Merge pull request #5 from arax/ref

Code clean-up, minor updates