Extra options for tags and dnf support

[Mike McLean]

With mock switching to dnf, we needed to conditionally adjust mock's
package_manager config on a per-tag basis. I didn't want to add yet another
specialized field in the tag_config table, so I extended the schema to
support more flexible extra options for tags.

This set of patches adds the extra options functionality and uses it to control
set mock's package_manager option.

Each tag can now be assigned arbitrary extra options. These are stored as json
in the database, so they can be of any type that json supports. The
getBuildConfig call follows inheritance when determining these.

To tell mock to use dnf in a tag, you would run a command like:

# koji edit-tag TAG -x mock.package_manager=dnf

This setting is inherited.