Support pip-style versions in *.yaml

Anvil failed on Centos 6.4 with message:

DependencyException: Pip prettytable==0.7.2 is already installed and it is not compatible with desired pip prettytable==0.6.1

The thing was that conf/distros/rhel.yaml required

         - name: prettytable
            version: "0.6.1"

It was not possible to set the version as ">=0.6,<0.8" (as OpenStack pip-requires do).

I propose to switch to pkg_resources.Requirement class for support for such version requirements.