HADOOP-10928. Incorrect usage on hadoop credential list. Contributed by Josh Elser.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1615828 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10927. Fix CredentialShell help behavior and error codes. Contributed by Josh Elser.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1615829 13f79535-47bb-0310-9956-ffa450edef68