BugFix: Check if ZK data dir is writable
BugFix: Check if ZK data dir is writable

- Fix for a bug, where
  ./onos.sh setup failed when ZK data dir already exists,
  but user did not have the write permission.

Change-Id: Ib0c303fdd5fc5e4d4eddaab80a95e3aa5fdbda9b

opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open