Support deprecated extensions/v1beta1 workload resources

[briandealwis]

Kubernetes 1.9 deprecated workload objects in extensions/v1beta1, apps/v1beta1, and apps/v1beta2. Skaffold continues to support workload objects in apps/* but not extensions/*. Since extensions/* were deprecated at the same time, and are supported on GKE, we should support their use.

Note that debug doesn't support them but does warn.

[codecov]

Codecov Report

Merging #4478 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4478      +/-   ##
==========================================
+ Coverage   72.31%   72.32%   +0.01%     
==========================================
  Files         333      333              
  Lines       12949    12949              
==========================================
+ Hits         9364     9366       +2     
+ Misses       2989     2988       -1     
+ Partials      596      595       -1     

Impacted Files	Coverage Δ
pkg/skaffold/deploy/kubectl/visitor.go	95.83% <ø> (ø)
...affold/kubernetes/portforward/kubectl_forwarder.go	63.41% <0.00%> (+2.43%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a46b873...6f2f1e3. Read the comment docs.