[neon] CIs: stricter versioning control

[Harald Sitter]

Since the debian version epoch keeps getting messed up every once in a
while in the CI branches I have now rolled out tech to prevent debian
epoch bumps (basically bumping the super overriding version of
packaging).

Our jobs now record the versions they built in a file called
'last_version', this file is read on subsequent runs and it is made
sure that the epoch version of the new build is not different from the
last one (no increments nor decrements allowed).

If the epoch is found to be different this will result in an unhandled
exception in the log looking a bit like this:

> Error: test_increment_fail(CI::VersionEnforcerTest): CI::VersionEnforcer::UnauthorizedChangeError:  -> 1

-->
The only way to get an epoch version change is to have someone
manually remove the last_version file from the job workspace!
<--

Usually only a jenkins admin will be able to do that. When in doubt, ask me.

HS