ACCUMULO-2194 Add delay for randomwalk Security teardown

If two Security randomwalk tests run back-to-back, the second test may see that the
table user still exists even though it was removed when the first test was torn down.
This can happen if the user drop does not propagate through Zookeeper quickly enough.
This commit adds a delay to the end of the Security test to give ZK some time.
Merge branch '1.4.5-SNAPSHOT' into 1.5.2-SNAPSHOT

Conflicts:
	src/server/src/main/java/org/apache/accumulo/server/test/randomwalk/security/SecurityFixture.java
Merge branch '1.5.2-SNAPSHOT' into 1.6.0-SNAPSHOT
Merge branch '1.6.0-SNAPSHOT'