subsequent runs of xtradb show no changes/errors when there are none.

Clean up tasks which where showing up as 'changed' in
percona tasks, which weren't actually changed, by using
`changed_when:`.

The intention is to have the entire site.yml result in 0
changes reported after the subsequent install, as changes
can be an indication of something going wrong, and these
alerts are masked when the run always has yellow tasks.

The first commit introduces a dependency on ansible 1.3, to use
the changed_when keyword.

The second commit updates `bin/install-$OS` to pick up ansible 1.3,
instead of 1.2, as they were doing before.