cleanup common flags + better -f description

[balopat]

I wanted to rename -f flag's description to Path or URL to the Skaffold config file.
However I found that we had a bunch of duplication - so I cleaned up that duplication.
In the process, I had to add CommonFlags to diagnose, fix and init. It turns out we wanted diagnose to take -c as well, so I left it there.

[codecov]

Codecov Report

Merging #3786 into master will increase coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/flags.go	100% <ø> (ø)	⬆️
cmd/skaffold/app/cmd/diagnose.go	27.77% <100%> (-7.23%)	⬇️
cmd/skaffold/app/cmd/init.go	100% <100%> (ø)	⬆️
cmd/skaffold/app/cmd/fix.go	67.74% <100%> (ø)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	68.29% <0%> (+2.43%)	⬆️

[briandealwis]

My $0.02: init and fix would benefit from a different -f text with an indication that the file will be changed -- the other cases the file is only read.

[balopat]

My $0.02: init and fix would benefit from a different -f text with an indication that the file will be changed -- the other cases the file is only read.

I think that's a fair point, I did think about that too. I think for fix we are good - as it's an input primarily - but for init we are generating it.

[briandealwis]

Agreed with fix: --overwrite does say that it overwrites the original with fixed content.