prefer createrepo_c if available

[cheese]

createrepo_c is a faster and more maintained version than the original python implementation, a.k.a createrepo. Both provide compatible command line options.

createrepo_c is available in official CentOS 7 repo.

Planex has a heavy use of createrepo. createrepo_c will certainly imporve performance of planex.

And on Fedora 26 with planex 2.1.0, tmp repo created by Python 'createrepo' is not usable by the current version of dnf (dnf-2.7.5-1.fc26.noarch).

[euanh]

After we move to Python 3 we can use shutil.which() to test for the existence of createrepo_c. Without that it will be a bit uglier.

[mseri]

I'll postpone this to after the move to python3 then