Introduces image build for bionic with py3 and nautilus.
Switches centos to py3 and fixes related issues.
Xenial is now run with stable/ocata.
Adds ability to skip specific build for particular component specifying
"supported_releases" in zuul component configuration. Sets
supported_releases for placement not available for releases prior to
Stein.
Removes redundant parameters for project reference using defaults
instead, as they relate to loci project, not to project we build
with loci. Also hard-codes parameters in playbooks/vars.yaml, making
it a definitive build configuration.
Change-Id: I7290203d80461f46ed70d5e3bea55c9765f9e746
d5ff58a973