Install varnish(package) from varnish(yum) repo.

On Amazon AMI, varnish 3.0 package is being installed from Amazon repo
due to priorities. This change forces the package to be installed from
varnish repo which is being added by this role.

epel repo is needed for other dependencies.
Merge pull request #4 from sredhar/sridhar/amazonami-fix

Add varnish repository to 'enablerepo' explicitly to prevent version problems on AWS.