Revert "Use BuildRequires tag in model spec files"

This reverts commit 85fe4525ae1e68e32008b0edc1bd9795c6717b0d.
Implement a workaround for BuildRequires tag im model spec files
The solution is to include a "build_requires.txt" file in the model
directory that lists the packages for the BuildRequires tag. The
list is passed to rpmbuild as a macro.

This workaround is necessary because Travis CI only supports an
Ubuntu build environment, which causes rpmbuild to fail because the
package names on Ubuntu are different.
Update dependencies files in config/
And only use the "_buildrequires" macro on RHEL-compatible
systems.