Fix BrooklynMementoPersisterToMultiFileTest.tearDown

- stop management context before deleting mementoDir, otherwise the management context can be reading/writing to dir at same time as we are trying to delete its contents.

See https://builds.apache.org/job/incubator-brooklyn-pull-requests/org.apache.brooklyn$brooklyn-core/407/consoleFull

```
2014-11-25 23:37:18,300 INFO  TESTNG PASSED CONFIGURATION: "Surefire test" - @BeforeMethod brooklyn.entity.rebind.persister.BrooklynMementoPersisterTestFixture.setUp() finished in 214 ms
2014-11-25 23:37:18,300 INFO  TESTNG INVOKING: "Surefire test" - brooklyn.entity.rebind.persister.BrooklynMementoPersisterTestFixture.testLoadAndCheckpointRawMemento()
2014-11-25 23:37:18,301 INFO  TESTNG SKIPPED: "Surefire test" - brooklyn.entity.rebind.persister.BrooklynMementoPersisterTestFixture.testLoadAndCheckpointRawMemento() finished in 1 ms
org.testng.SkipException: Persister brooklyn.entity.rebind.persister.BrooklynMementoPersisterToMultiFile@f2fee1 not a BrooklynMementoPersisterToObjectStore
	at brooklyn.entity.rebind.persister.BrooklynMementoPersisterTestFixture.testLoadAndCheckpointRawMemento(BrooklynMementoPersisterTestFixture.java:163)
2014-11-25 23:37:18,302 INFO  TESTNG INVOKING CONFIGURATION: "Surefire test" - @AfterMethod brooklyn.entity.rebind.persister.BrooklynMementoPersisterToMultiFileTest.tearDown()
2014-11-25 23:37:18,318 INFO  TESTNG FAILED CONFIGURATION: "Surefire test" - @AfterMethod brooklyn.entity.rebind.persister.BrooklynMementoPersisterToMultiFileTest.tearDown() finished in 0 ms
brooklyn.util.exceptions.PropagatedRuntimeException: java.io.IOException: Unable to delete '/tmp/BrooklynMementoPersisterToMultiFileTest-OAJF': delete returned false
	at brooklyn.util.exceptions.Exceptions.propagate(Exceptions.java:91)
	at brooklyn.util.os.Os$DeletionResult.throwIfFailed(Os.java:278)
	at brooklyn.util.os.Os$DeletionResult.asNullOrThrowing(Os.java:285)
	at brooklyn.entity.rebind.persister.BrooklynMementoPersisterToMultiFileTest.tearDown(BrooklynMementoPersisterToMultiFileTest.java:57)
Caused by: java.io.IOException: Unable to delete '/tmp/BrooklynMementoPersisterToMultiFileTest-OAJF': delete returned false
	... 33 more
Caused by: java.io.FileNotFoundException: File does not exist: /tmp/BrooklynMementoPersisterToMultiFileTest-OAJF/enrichers/IE5jOu72.tmp
	at org.apache.commons.io.FileUtils.forceDelete(FileUtils.java:2275)
	at org.apache.commons.io.FileUtils.cleanDirectory(FileUtils.java:1653)
	at org.apache.commons.io.FileUtils.deleteDirectory(FileUtils.java:1535)
	at org.apache.commons.io.FileUtils.forceDelete(FileUtils.java:2270)
	at org.apache.commons.io.FileUtils.cleanDirectory(FileUtils.java:1653)
	at org.apache.commons.io.FileUtils.deleteDirectory(FileUtils.java:1535)
	at brooklyn.util.os.Os.deleteRecursively(Os.java:239)
	at brooklyn.util.os.Os.deleteRecursively(Os.java:225)
	... 31 more
```