add priorities option to rhel repo

yum plugin priorities support. I didn't want to rewind the template. Possibly add a template option to the LWRP so we can modify a template in our wrapper cookbooks ?  

This PR is more about having a discussion/raising an issue. I also needed this to work today so I hacked this into my fork. 
Probably better to default to false since I don't think this option is very common. Also, please add a space before `TrueClass`.