Issue #339: mock build hangs with f26 mock config - koji

Today, I used koji to generate a f26 mock config file

koji mock-config --tag f26-build --arch=x86_64 --topurl=http://kojipkgs.fedoraproject.org/ -o f26-x86_64.cfg

and met an issue that following line does not appear in the config as well as repos

config_opts['package_manager'] = 'dnf'

BTW, after I added this line manually,

mockbuild hangs at step

Start: dnf install

A few notes:

part of the code that handles this for builders is not in koji.getMockConfig, but in the BuildRoot class from kojid
short term, we just need to have the cli handler do the same thing:
- fetch the tag build config at the correct event (session.getBuildConfig(tag_id, event=event_id))
- if mock.package_manager is set in the tag extra data, then pass appropriate package_manager option to genMockConfig
longer term we should probably unify some of this code, but that may require some api changes in the library