Support for multiple VO in keystone

If you get multiple tenants from keystone, instead of selecting just the first one, select the first one which gives you a valid token
[AuthN_KS] Cleaned up some old KS code

* DRYing up headers
* Full integration of the patch from @spinto
* Formatting changes
Minor version bump for rOCCI-core to v4.2.16
Minor version bump to v4.2.6

* Updated Keystone AuthN plugin (@spinto, tenant selection)
* New rOCCI-core (improved parsing of custom resources)