heisenbug in CleanUpActivationsTest

Failed once in this [build log](https://api.travis-ci.org/jobs/272670174/log.txt?deansi=true).

```
whisk.core.database.test.CleanUpActivationsTest > Database clean up script should delete old activations and keep new ones FAILED
org.scalatest.exceptions.TestFailedException: Left(404 Not Found) was not right
at org.scalatest.MatchersHelper$.indicateFailure(MatchersHelper.scala:340)
at org.scalatest.Matchers$AnyShouldWrapper.shouldBe(Matchers.scala:7189)
at whisk.core.database.test.DatabaseScriptTestUtils$$anonfun$removeDatabase$1.apply(DatabaseScriptTestUtils.scala:87)
at scala.util.Try$.apply(Try.scala:192)
at whisk.utils.retry$.apply(Retry.scala:44)
at whisk.core.database.test.DatabaseScriptTestUtils$class.retry(DatabaseScriptTestUtils.scala:48)
at whisk.core.database.test.CleanUpActivationsTest.retry(CleanUpActivationsTest.scala:44)
at whisk.core.database.test.DatabaseScriptTestUtils$class.removeDatabase(DatabaseScriptTestUtils.scala:85)
at whisk.core.database.test.CleanUpActivationsTest.removeDatabase(CleanUpActivationsTest.scala:44)
at whisk.core.database.test.CleanUpActivationsTest$$anonfun$1.apply(CleanUpActivationsTest.scala:107)
at whisk.core.database.test.CleanUpActivationsTest$$anonfun$1.apply(CleanUpActivationsTest.scala:81)

```

Start with CleanUpActivationsTest.scala, especially the test around line 107, and follow DatabaseScriptTestUtils.removeDatabase at line 85 and its retry path. Review the linked Travis build log to understand the intermittent Left(404 Not Found); done means the cleanup test runs reliably while preserving its expected old-versus-new activation behavior.

Written by the indexing model from the issue text.