Fix security_available method in security.rb

"exists?" method expects parameter tableName to be String but ACL_TABLE_NAME is of org.apache.hadoop.hbase.TableName form.

raise(ArgumentError, 'DISABLED: Security features are not available') unless \
          exists?(org.apache.hadoop.hbase.security.access.AccessControlLists::ACL_TABLE_NAME.getNameAsString)

Impact of the bug:-
So , if a user is running any security related command(revoke,user_permission) and there is an exception(MasterNotRunning) while checking security capabilities, then instead of seeing the underlying exception, user is seeing

ERROR: no method 'valueOf' for arguments (org.apache.hadoop.hbase.TableName) on Java::OrgApacheHadoopHbase::TableName
  available overloads:
    (java.lang.String)
    (byte[])