debian: Don't overwrite changelog when OBS bumps the version

OBS adds an additional version suffix at the beginning of the build in
debian/changelog. This breaks the Ubuntu flow where all real changes go
into debian.master/changelog. Likewise, the version detection in the
rules (unlike the debian packaging tools) read debian.master/changelog.
We need to preserve the OBS version, so keep the two files in sync
during clean.

[endlessm/eos-shell#2600]