Bug/retries in failure authentication

Redo the retries in Keystone calls to avoid making retries for every error. Now there is a selection based on the error name. Currently, only "expired token" errors are being retried, we would add more as we see them fit. Some of the error checking from keystone were changed to clearly disntinguish between authentication problems (token expired) and other errors.

Also fixes an small problem on the environment variable-based configuration of the plugins.
The definition of the error in the "errors.js" file is 'KEYSTONE_PROXY_AUTHENTICATION:REJECTED' where is defined 'PEP_PROXY_AUTHENTICATION_REJECTED'?