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
