Add support for StatefulSets

[vbehar]

With this change, Osiris can now manage both Deployments and StatefulSets.

I introduced a new service annotation osiris.deislabs.io/statefulset to link a service with its statefulset, otherwise everything works the same as with Deployments.

Internally, I tried to avoid duplicated code, and introduced a kind parameter to make a distinction between deployments and statefulsets.

[codecov-io]

Codecov Report

Merging #43 into master will decrease coverage by 0.65%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage    57.3%   56.65%   -0.66%     
==========================================
  Files          11       11              
  Lines         623      639      +16     
==========================================
+ Hits          357      362       +5     
- Misses        237      248      +11     
  Partials       29       29

Impacted Files	Coverage Δ
pkg/kubernetes/informers.go	0% <0%> (ø)	:arrow_up:
pkg/net/http/httputil/reverseproxy.go	73.01% <0%> (+1.98%)	:arrow_up:

---

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 113a458...fd733a4. Read the comment docs.

[krancour]

@vbehar this has some merge conflicts after support for shorter DNS names was merged.

[vbehar]

yes thanks, I'll fix them

[vbehar]

ok, rebased on master

[krancour]

Need rebase after recent merge.

[vbehar]

@krancour ok, it's been rebased after the recent merges.