xci: Decouple VM distribution from host OS

xci: Decouple VM distribution from host OS

Introduce a new XCI_DISTRO variable to select the distribution to deploy
on the VMs in order to make deployments more flexible and decouple the
VM OS selection from the host one. The default value for this new
variable is to match the host OS but users can always set it to one
of the supported distributions. We can now simply execute the
install-ansible.sh script instead of sourcing it in order to keep
the environment as clean as possible.

Change-Id: Ia74eb0422f983848cde0fb7b220ea1035dfa78bc
Signed-off-by: Markos Chandras <mchandras@suse.de>