Make use of cache in scaling

- this reinstates the cache to reduce the count of lookups to the
provider when checking if scaling is needed.

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

Extract scaling from zero

- extracting this package means it can be used in other components
such as the asynchronous nats-queue-worker which may need to
invoke functions which are scaled down to zero replicas.

Ref: openfaas/nats-queue-worker#32

Tested on Docker Swarm for scaling up, already scaled and not
found error.

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

Update for scaling edge-case

- as reported on Slack and in issue openfaas#931 the gateway scaling code
was scaling to zero replicas as a result of the "proportional
scaling" added by @Templum's PR. This commit added a failing test
which was fixed by adding boundary checking - now if the scaling
amount is "0" we keep the current amount of replicas.

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

Update for scaling edge-case

- as reported on Slack and in issue openfaas#931 the gateway scaling code
was scaling to zero replicas as a result of the "proportional
scaling" added by @Templum's PR. This commit added a failing test
which was fixed by adding boundary checking - now if the scaling
amount is "0" we keep the current amount of replicas.

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

Update for scaling edge-case

- as reported on Slack and in issue openfaas#931 the gateway scaling code
was scaling to zero replicas as a result of the "proportional
scaling" added by @Templum's PR. This commit added a failing test
which was fixed by adding boundary checking - now if the scaling
amount is "0" we keep the current amount of replicas.

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

Update CONTRIBUTING.md

Change the http status code on unfound function error to 404

Signed-off-by: Richard Gee <richard@technologee.co.uk>

Enable basic auth for service query / scaling on provider

- this is a blocking issue for auth with Docker Swarm
fixes openfaas#879

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

Alter graceful shutdown sequence

- the shutdown sequence meant that the kubelet was still passing
work to the watchdog after the HTTP socket was closed. This change
means that the kubelet has a chance to run its check before we
finally stop accepting new connections. It will require some
basic co-ordination between the kubelet's checking period and the
"write_timeout" value in the container.

Tested with Kubernetes on GKE - before the change some Pods were
giving a connection refused error due to them being not detected
as unhealthy. Now I receive 0% error rate even with 20 qps.

Issue was shown by scaling to 20 replicas, starting a test with
hey and then scaling to 1 replica while tailing the logs from the
gateway. Before I saw some 502, now I see just 200s.

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

Fix issue with direct_functions and path behaviour

- The path clipping / transforming behaviour must be turned-off
when we are not using direct_functions as is used in
faas-nomad and faas-ecs. This will need a change in each provider
to strip paths, but fixes a 404 error these users will see if they
upgrade to 0.9.2 or newer. 0.9.3 will have a this fix meaning
the whole un-edited path is passed to the provider when
direct_functions is set to false.

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