Fix writing rendered manifests to files

[nkubala]

before this change, trying to run skaffold render --output file.txt would fail with

FATA[0001] rendering manifests: opening file for writing manifests: open file.txt: no such file or directory

this was happening because we were trying to do an os.Open() on a file that didn't exist. now, we'll just do an os.OpenFile() with the right opening mode, and just call file.WriteString() rather than creating a new Writer.

[codecov]

Codecov Report

Merging #3152 into master will decrease coverage by <.01%.
The diff coverage is 40%.

Impacted Files	Coverage Δ
pkg/skaffold/deploy/kubectl.go	63.97% <40%> (-0.21%)	⬇️