Do not mess with scl repos if package is already available

[jflemer-ndp]

Sometimes packages and yum repos are managed outside the "standard" mechanisms (e.g. RHSM). The existing SCL code (at least for RHEL) always insists on checking for and enabling a specific repo vis RHSM.

This patch and supports alternate repo strategies by checking to see if the package we want is already available (in Chef's YUM cache) before messing with repos. This also happens to avoid excess YUM cache flushes. So as long as a prior cookbook/recipe or existing config makes the appropriate RPM available, now poise won't mess with (and possibly break) yum repos/subscriptions.

[jflemer-ndp]

cc @jmercier-ndp

[jflemer-ndp]

There is a small assumption here, that if the primary scl package is available, then so is the devel package. But a similar assumption already exists in the install_scl_repo method that a single repo has both packages.