Keep Ubuntu repository indices for Fuel ISO.

It is useful for further manipulation with Ubuntu
repository placed on the Fuel master node.

Implements: https://blueprints.launchpad.net/fuel/+spec/testing-packages-in-enviroment

Change-Id: I8f0e1fc11a3ee58c46fbd9f44e00da7bb2f91c0f
Merge "Keep Ubuntu repository indices for Fuel ISO."