authentication: register global shared session key used for external authentication session data storeage.

- We'll have more plugins not only oauth so this should be a constant name, instead of repating the same name all over the code.

rhodecode/authentication/base.py

@@ -53,6 +53,8 b' log = logging.getLogger(__name__)'
 VCS_TYPE = 'vcs'
 HTTP_TYPE = 'http'
 
+external_auth_session_key = 'rhodecode.external_auth'
+
 
 class hybrid_property(object):
     """