IMPALA-4702: Fix command line help for webserver_private_key_file
IMPALA-4702: Fix command line help for webserver_private_key_file

The command line help for 'webserver_private_key_file' should refer to
'webserver_certificate_file' instead of 'ssl_server_certificate'.

Change-Id: Ie2820951b381240d5dab7c98e07e92332234462a
Reviewed-on: http://gerrit.cloudera.org:8080/5552
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Internal Jenkins

michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala