We current require Java 8 to install the scylla-jmx package on Debian.
As Debian 10 defaults to Java 11, let's remove the conflict flag and add
Java 11 to the dependencies list.
Debian package builds provide a root environment for the installation
scripts, since that's what typical installation scripts expect. To
avoid providing actual root, a "fakeroot" system is used where syscalls
are intercepted and any effect that requires root (like chown) is emulated.
However, fakeroot sporadically fails for us, aborting the package build.
Since our install scripts don't really require root (when operating in
the --packaging mode), we can just tell dpkg-buildpackage that we don't
need fakeroot. This ought to fix the sporadic failures.
As a side effect, package builds are faster.
Follows scylla.git's b608af870b0a1ad88b91a72bddeff0c321877f9e.
Refs scylladb/scylla#6655.
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
