Substantiate failing for db_lp

Recently the db_lp slave has not been substantiating successfully. The error logs on the master are not very useful because there is an additional error during reporting - https://pastebin.canonical.com/24198/ - which masks the cause.

Gary Poster looked at the log leading up to the error - https://pastebin.canonical.com/24199/ - and commented:

 - The instance successfully starts. It does it in 55 seconds, which
   seems fast to me historically. I'd double check that devel is
   starting that quickly.

 - Once it starts, it doesn't ever talk. In the past, this was
   generally indicative of pid or firewall issues, per my earlier
   comment.

 - To diagnose further, I would start up an instance of the image and
   give it the expected IP for the db-devel slave. I would then see if
   buildbot started at all, and try to get to the buildbot master in
   the data center.