(RE-1736) Update pbuilderrc for cumulus oddities

Because we have no public apt repos for cumulus, the OTHERMIRROR setting
must be unset here, as it has been previously set to apt.pl.com...
Additionally, the build-tools repo is signed with the pluto keyring, so
it must be present for the cow to be able to install the required
packages from the repos.
(RE-1736) Hack around cumulus cl-basefiles bug in debootstrap

cl-basefiles tries to restart openssh-server during the clinit start of
its init script. openssh-server is not listed in its dependencies, so
this fails, causing the cow itself to fail to build. This commit works
around that problem by manually installing openssh-server during the
debootstrap process, immediately after the core packages are installed
and before the 'required' packages like cl-basefiles are installed.