Check if mandatory secrets/configmaps exist

[Nerja]

This PR adds checks for errors when fetching secrets and config maps to make sure that an unavailable API server will not cause Flagger to consider it a diff and trigger a new canary deployment.

[codecov-io]

Codecov Report

Merging #799 (ec73a53) into main (118cac5) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #799      +/-   ##
==========================================
+ Coverage   56.61%   56.84%   +0.22%     
==========================================
  Files          67       67              
  Lines        5403     5401       -2     
==========================================
+ Hits         3059     3070      +11     
+ Misses       1882     1873       -9     
+ Partials      462      458       -4     

Impacted Files	Coverage Δ
pkg/canary/config_tracker.go	83.64% <100.00%> (+5.86%)	⬆️

---

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 118cac5...af1614e. Read the comment docs.

[stefanprodan]

Deployments can reference non existing Secrets and ConfigMaps https://github.com/kubernetes/kubernetes/blob/fc39904bed3b935f8989b7064e696a5c08ce77b3/pkg/apis/core/types.go#L1904 we need to error out only if the optional fields is set to false.

[Nerja]

@stefanprodan Changed the implementation so that we keep the old behavior if optional is not set, or set to true. We will error out if we encounter a secret/config ref with the optional flag set to false.

[stefanprodan]

LGTM

Thanks @Nerja