spec: require python-argparse on rhel6
spec: require python-argparse on rhel6

Closes #60.

jpopelka/osbs-client,bfontecc007/osbs-client,DBuildService/osbs-client,jpopelka/osbs-client,projectatomic/osbs-client,bfontecc007/osbs-client,vrutkovs/osbs-client,DBuildService/osbs-client,projectatomic/osbs-client,vrutkovs/osbs-client