backend: migrate to nova ansible module

because ec2 module does not work in F20
and ansible upstream is reluctant to work on that:
https://github.com/ansible/ansible/pull/7603
Automatic commit of package [copr-backend] release [1.36-1].
Automatic commit of package [copr-cli] release [1.33-1].
branch is epel7, not el7
document vm_name option