STS user impersonation - EUCA-8496

EUCA-8496
Add token service action for use with user console - EUCA-8502

A new eucalyptus specific GetAccessToken action is added to the tokens
service. Tokens issued by this action are suitable for use with IAM
actions.

EUCA-8502
STS user impersonation, fix action - EUCA-8496

EUCA-8496
Merge remote-tracking branch 'origin/dev/sjones/EUCA-8502' into testing