Update release vars functions

Release schema verification needs to happen
first before we attempt to assign values to
the variables.

Organize variable setup into functions.
Maven release files expects different variables
than container release files.

Rename "version" variable in container release
files to "container_release_tag" which is a
better user friendly name given the fact that
container versions are rather called tags.
Internally, we still process it as "version"
to allow reuse of the tag function.

Issue: RELENG-2353
Change-Id: Ie0456a1fa87ea62855cb5d1140f6ae9f32b3e566
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>