fixes #5100 - updates to how/when foreman content is created

This commit addresses 2 issues:

1. Do not create foreman content (install media, OS...etc) when creating a repo in the default content view (i.e. Library)
2. Do not blindly create install media, if it already exists (e.g. rather than Medium.create! do Medium.find_or_create_medium)
Merge pull request #3954 from bbuckingham/issue-5100

fixes #5100 - updates to how/when foreman content is created