Fix replicalimits ConstraintTemplate to handle scaling to zero

[skaven81]

What this PR does / why we need it:
The replicalimits ConstraintTemplate in the library had no unit tests for validating scaling to zero.

It turns out that when scaling to zero, Kubernetes generates a Scale resource with an empty spec, instead of spec.replicas=0.

The unit tests were also missing tests using the Scale resource.

This PR fixes both issues.

Which issue(s) does this PR fix
Fixes #419

[skaven81]

Additional unit tests confirm that the updated template works as expected.

=== RUN   replica-limit
    === RUN   example-allowed
    --- PASS: example-allowed	(0.005s)
    === RUN   example-scale-allowed
    --- PASS: example-scale-allowed	(0.006s)
    === RUN   example-disallowed
    --- PASS: example-disallowed	(0.004s)
    === RUN   example-scale-disallowed
    --- PASS: example-scale-disallowed	(0.005s)
--- PASS: replica-limit	(0.027s)
=== RUN   replica-limit-zero
    === RUN   example-allowed
    --- PASS: example-allowed	(0.008s)
    === RUN   example-scale-allowed
    --- PASS: example-scale-allowed	(0.006s)
    === RUN   example-disallowed
    --- PASS: example-disallowed	(0.006s)
    === RUN   example-scale-disallowed
    --- PASS: example-scale-disallowed	(0.005s)
--- PASS: replica-limit-zero	(0.031s)
ok	suite.yaml	0.058s
PASS

[apeabody]

Thanks for the contribution @skaven81!

You will also need to make your template changes to https://github.com/open-policy-agent/gatekeeper-library/blob/master/src/general/replicalimits/constraint.tmpl and https://github.com/open-policy-agent/gatekeeper-library/blob/master/src/general/replicalimits/src.rego. The artifacts (e.g. template.yaml can then be built with make generate-all.

[skaven81]

I'm not sure what I borked here, but I'll close this and try again with a fresh branch

[skaven81]

Opened #427 with corrected branch contents