Make Amazon Linux use yumpkg5 provider

Due to a bug in the rpm python API, Amazon Linux is unable to retrieve
repo metadata over SSL connections. This commit forces Amazon Linux to
use yumpkg5 while we decide what to do about this.

See the following comment from the issue tracker for more info:
https://github.com/saltstack/salt/issues/8226#issuecomment-29664936
Merge pull request #8958 from terminalmage/issue8226

Make Amazon Linux use yumpkg5 provider