Configure metadata source for cloud-init

When building Ubuntu images the metadata source to be used by
cloud-initmust be explicitly set.

Change-Id: I30e3336cf65f8c1baed358efd78d90cde1d0cc02
Closes-Bug: #1250881
backport: havana
(cherry picked from commit d0dc1ead6b3cc567948524ceed0cc7b48b912f7b)