wikistats - ensure php5-mysql is installed

before we got this from webserver::php5 afair

now, when creating a fresh labs instance, we don't get
it anymore, so adding it explicitely or fail:

"PHP Fatal error:  Call to undefined function mysql_escape_string() "

which is breaking it

Change-Id: I0935892a9fc32ab48a90071673b5a8017bbd19fa