buildpackages: wait for the build machine deletion

buildpackages: wait for the build machine deletion

When the quotas are low, it matters to block until the build machine is
actually deleted. Otherwise target provisionning may fail because the
they exceed the quota. For instance the default on OVH is to have 32
cores and the build machine uses 16. The packages-repository machine
uses two, the teuthology cluster uses one and that leaves only 13 cores
for the targets which may be too low when running jobs that require
large instances.

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