Instead of using the baked-in values from SCYLLA-VERSION-GEN,
allow passing an override. This will be used by the supermodule
to have an identical product-version-release (especially release,
which contains the git hash) across all packages.
Drop dependency on pystache since it nolong present in Fedora 32.
To implement it, simplified debian package build process.
It will be generate debian/ directory when building relocatable package,
we just need to run debuild using the package.
To generate debian/ directory this commit added debian_files_gen.py,
it construct whole directory including control and changelog files
from template files.
Since we need to stop pystache, these template files swiched to
string.Template class which is included python3 standard library.
see: https://github.com/scylladb/scylla/pull/6313
To align build system with scylla main repo, adding relocatable package
support.
On scylla-jmx, we don't provide libraries and linker since it's Java
program, just contains .jar file and dist/ directory.
