Set default python3 version as 3 for cases python3
is not installed before running DevStack.
Implements installation of required python3x package
for RedHat family distros with package name depending on
configurable ${PYTHON3_VERSION}. Examples:
3 => python3 python3-devel (default one)
3.6 => python36 python36-devel
3.7 => python37 python37-devel
This should help in situations where there are more
than one python available for given platform and
DevStack is asked to pick one by its full 3.x version
Change-Id: I49d86bc9193165e0a41e8e8720be37d81a4e7ee0
2dcbc28aba