Add public alias for config-master.

As it is sometimes handy to be able to fetch pybal configs from outside
the internal network, and it's also needed by various scripts, we create
a public alias for config-master on misc-web-lb.

Change-Id: I8143aa88f3f91de37130a2f6587676175ba85a17
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>