Fix RPM package release and version files expansion

[k0walik]

What

While trying to migrate from version 0.9.1 to 0.10.0 I stumbled across following error:

Traceback (most recent call last):
        File "/home/tomasz.wojno/code/rules_pkg/pkg/rpm_pfg.bzl", line 352, column 47, in _pkg_rpm_impl
                content = substitute_package_variables(ctx, "\n".join(preamble_pieces)),
        File "/home/tomasz.wojno/code/rules_pkg/pkg/private/util.bzl", line 95, column 34, in substitute_package_variables
                return attribute_value.format(**vars)
Error in format: Missing argument 'VERSION_FROM_FILE'

In #787 a helper function for expanding RPM preamble with user-defined variables was introduced. The function uses Starlark format expression to replace {var} occurrences with defined variables. However, this does not work when we pass release or version file to the build target. The rpm_pfg.bzl appends Release: ${RELEASE_FROM_FILE} and Version: ${VERSION_FROM_FILE} to preamble, which are later expanded by make_rpm.py script. Thus, those variables are failed to be expanded as they are not defined.

Changes

This pull request introduces following changes:

• Avoid expanding RELEASE_FROM_FILE and VERSION_FROM_FILE by wrapping them in double curly braces. This way ${{RELEASE_FROM_FILE}} becomes expanded to ${RELEASE_FROM_FILE}, so it can be processed later by make_rpm.py
• Add tests that uses version_file and release_file attributes
• Fix passing changelog file to OutputGroupInfo

[cgrindel]

@aiuto Do you want to review or can I merge this PR?