Added cache for image_username

This change removes roundtrip to nova for each remote() call.

It is also important for long provision operations because auth
token could expire during cluster provisioning timeframe.

Introduced nodegroup.image_username field with information about
username used.

Change-Id: I00f318a9bf34b221c8ccf7a4f22e9ed3fd91e806
Merge "Added cache for image_username"