Katello: check validity of iso cdn urls

Commit f0581b72a63bd9c4aca36f62c59523097fa338b9 introduced
the ability to check if content urls are valide and throw
an error if they are not. Unfortunately the validity checks
did not work correctly with iso content, as iso content sets
do not have yum repodata. In case there is no repodata we
need to check if there is a pulp_manifest before declaring
the urls invalid.
Issue 5037: Can now enable ISO repo sets from a manifest

See http://projects.theforeman.org/issues/5037
Previously the MetadataGenerate could not correctly handle
repositories with "file" content types (ISO repo sets). This
prevented you from ever enabling or syncing an ISO repo set.
We just have to update MetadataGenerate to know the appropriate
distributor class for "file" content types.