Cleanup of build-time shell variable replacements

Instead of having a different set in (almost) every shell script, this
inserts the most commonly used variables at build time. This way the
code for injecting a root directory for virtual clusters also is just
needed once.

I chose inserting the variables using “sed” at build time over a runtime
script as the latter would need to be located from within the script and
the path would be different depending on the environment (unittest vs.
actual installation).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>