This change adds the ability to use the k8s (and friends) drivers
to create pods with custom specs. This will allow nodepool admins
to define labels that create pods with options not otherwise supported
by Nodepool, as well as pods with multiple containers.
This can be used to implement the versatile sidecar pattern, which,
in a system where it is difficult to background a system process (such
as a database server or container runtime) is useful to run jobs with
such requirements.
It is still the case that a single resource is returned to Zuul, so
a single pod will be added to the inventory. Therefore, the expectation
that it should be possible to shell into the first container in the
pod is documented.
Change-Id: I4a24a953a61239a8a52c9e7a2b68a7ec779f7a3d
c78fe769f2