(PUP-2728) Remove rug package provider as default

We only support SLES 10.4 which uses zypper, and when zypper is present
we should always use it over rug. This commit removes rug as a default
provider; if zypper is not present then rug should be used as a fallback
due to the number of confine statements.
Merge pull request #2917 from adrienthebo/issue/puppet-4/pup-2728-remove-rug-pkg-provider-default

(PUP-2728) Remove rug package provider as default