Fix lack of support for MetasploitV5 tag

 #4184

* Appears to have been overlooked somehow in the pre-BlackHat crunch
* V5 will not support credentials
* We are implementing full-workspace zip import/export for credentials

## Verification

- [x] Use a workspace that contains hosts/services/etc data
- [x] `db_export -f xml /tmp/msf-export.xml`
- [ ] `workspace -a import-test`
- [ ] `db_import /tmp/msf-export.xml`
- [ ] VERIFY: import completes and all data is persisted from original workspace to new one