[zypp] add service file to clean rpmdb files. Contributes to JB#17331

The service file is only installed with the -zypp backend package.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jolla.com>
Merge pull request #41 from kaltsi/rpmdb-cleanup

[zypp] add systemd service file to clean rmdb files. Contributes to JB#1...