[Drop Box] Permit Remove Manifestation in own drop box

Addresses root cause of nasa/mct#281 (details specified in Remove Manifestation policy were out of sync with details specified during automatic drop box creation) and takes small additional measures to reduce the likelihood that the problem might recur.

```
 Wrote or re-wrote unit tests (Y/N): Y
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Self-review by pull request
 Describe potential impact on other areas of the code: Automatic database initialization & user creation with Derby
 Describe any test prerequisites for this fix (e.g., restart the database): Run a new instance of MCT using Derby and automatically.add.user=true
 For bugs marked "Regression - Yes", describe the cause of the regression: N/A
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N
```