Midokura repo URLs should use RHEL 7 repo when using CentOS/RHEL7

currently it points to only rhel 6 repo,
https://github.com/eucalyptus/midokura-cookbook/blob/master/recipes/_common.rb#L20-L26
https://github.com/eucalyptus/midokura-cookbook/blob/master/attributes/default.rb#L11

Perhaps use something like this,

```
if Chef::VersionConstraint.new("~> 7.0").include?(node['platform_version'])
...
end
```
