buildpackages: refactor to not require remote

buildpackages: refactor to not require remote

Most of the flavor, sha1, tag etc. selection logic as implemented in the
packaging module of teuthology relies on remote hosts. This is complex
to tests and inconvenient because hosts must be provisionned even before
trying to figure out which packages need to be installed.

Using remote hosts is necessary when bare metal targets are used because
teuthology must adapt to the operating system already installed. The
selection logic in the context of dynamically provisionned targets is
simpler because it is defined by the job being run.

The buildpackages is refactored to use only the job configuration to
figure out which packages must be built. It makes it specific to targets
that are dynamically provisionned. It would have to be modified to query
the remote host in the case of bare metal targets.

Signed-off-by: Loic Dachary <loic@dachary.org>