verify config setting before repo export
In the planning step, confirm that the export dir setting is set, exists, and is writable.
Fixes #13885 - verify export destination is a directory before exporting
This patch makes two changes:
- it alters the pulp_export_destination setting to a setting that must be
filled in by the user prior to first export
- it adds a check to the repository export task to ensure the above setting is
set, and that the dir is writable.
Previously, users would perform an export and inadvertently fill up their tmpfs
space. There is not a good default production value for the export destionation
since an export could be multiple GB.