Introduce basic support for provisioning nodes with centos7

This change makes small adaptations to allow the use of bifrost
to provision nodes using centos7. Please note that engine must still
be executed on an ubuntu1804 node due to lack of proper python3
support on centos7.

The first update is done to configure-targethosts role, making sure
we run tasks based on the distro the nodes are provisioned with.

The other change makes it users to use prebuilt deployment image
by default to avoid unnecessary failures during diskimage builder
and also cuts the time it takes to provision nodes with.

Change-Id: I58965e979a2be099a0c88c328cc87dafd88d3c16
4 files changed