Refactor jobs for introduction of Centos7 support

Refactor jobs for introduction of Centos7 support

engine is currently being updated to introduce support for Centos7.
During that work, engine code has been refactored, aligning how things
are called and done when it comes to various configuration options based
on the distribution.

Engine now uses single variable to do things based on the distribution
selected by user using -o argument. This argument can take either ubuntu1804
or centos7. Based on that, we determine DIB stuff using vars files per
distro. This results in the removal of DIB parameters from jobs and DISTRIBUTION
parameter and only use DISTRO parameter which is then used by engine
accordingly.

Centos7 jobs for engine and scenario verification are enabled in a none-voting
fashion so if they fail due to development work, they will not block a change
from being merged. Only Ubuntu1804 jobs will vote until we prove Centos7 works
for all the scenarios we have.

Periodic jobs for engine and scenario are not enabled for Centos7 in order not
to waste the resources we have until Centos7 is proving to be working.

Change-Id: I09ab256ce1904b2c9d7b968e3be399ff82c099aa