Configures the Pod's termination grace period in seconds to
the write_timeout environment-variable, when available.
Otherwise, it's set to the default for Kubernetes, which is
30 seconds.

This change is important for allowing function containers
to drain active connections, without losing work in
progress.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Allow container registry to be overridden easily for
local builds

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

The additional 2s should prevent an issue where the grace
period is exactly the same as the timeout, and kills the Pod
before the remaining requests have completed.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>