skip_if_exists not passed to centos parser within RHEL parser

[wagoodman]

The RHEL parser uses the centos parser in order to download results. The centos parser has a skip_if_exists attribute that is leveraged by the centos provider based on the runtime config (wired into centos.Parser.get()). The same functionality in the RHEL parser is not wired to the internal centos parser instance even though there is a skip_if_exists feature for the RHEL provider as a whole.

Question: is leaving out skip_if_exists on the centos parser calls within the RHEL parser intentional? If so, we need some comments as to why to help folks out in the future.

[wagoodman]

Note: I've updated the provider to pass the skip_if_exists option in #19, however, I'm leaving this issue open since it is still technically an unanswered question