rpm: Fix make rpm on Debian/Ubuntu

The recent patch to change the bash completion install location based
on the Distribution, ignored that it should still be possible to
create RPMs on Debian derived systems. Additionally make deb itself
creates RPMs and converts them via alien.

This patch adds the bashcompletiondir variable to the rpm defines and
uses this for the location, where to get the bash completion file.

It still changes the location on Debian/Ubuntu systems in the final
packages from /etc/bash_completion.d to
/usr/share/bash-completion/completions

Fixes: e69ade32e116e72d03068c03799924c3f1a15c95

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Closes #15355
Closes #15365