Rename pulp_rpm.plugins.importers.iso_importer.

pulp_rpm.plugins.importers.iso_importer is renamed to
pulp_rpm.plugins.importers.iso so that the name isn't needlessly
redundant.
1025465 - Log all ISO download failures.
1065016 - Don't require optionlist to be present.

When processing repo groups, we used to require environments to have an
optionlist tag. This broke synchronization of EL 7 repositories, as
they appear to occasionally lack this tag. This commit removes the
requirement.
Merge branch 'rbarlow-1025465'