public static Set<String> parseHostFilter(final Map<String, String> requestProperties) {
    if (requestProperties.containsKey(DIRECTIVE_HOSTS)) {
      return ImmutableSet.copyOf(requestProperties.get(DIRECTIVE_HOSTS).split(","));
    }
    return null;
  }
Parsing 'Kerberos/hosts' property to get list of hosts for 'regenerate_keytabs' request.
Must be a string with coma separated list of hosts. Absent or miss-spelled hosts must be silently ignored
by caller code.

@param requestProperties
@return
ambari-server/src/main/java/org/apache/ambari/server/controller/KerberosHelperImpl.java