Fix os vg allocation on ubuntu

Ubuntu cant allocate more than one disk for os vg

In order not to introduce a lot of low quality fixes to volume manager,
revert os allocation on controller nodes to min size

This patch reopens: https://bugs.launchpad.net/fuel/+bug/1295717

Change-Id: I81ac8150d07fcf11c745159c42c6c2fa57d64b33
Closes-Bug: #1306625