add --dry-run option to skaffold run

[johnistan]

Feature Request:

When build new releases with complicated value files it would be nice to be to calculate and print the full manifest without actually deploying it to a cluster.

something like: skaffold run -p prod --dry-run

[nkubala]

A "true" dry run isn't currently possible, because image tag resolution is dependent on build results for some tagging strategies (mainly, the checksum tagger). If we don't care about skipping the image build (and pushing them to whatever remote) then in theory this should be possible now, but if we wanted to take it a step further, we could remove the checksum tagger and decouple the tagging step from the builders. Then, all image tags could be generated for each run before skaffold builds anything.

[alnr]

Note you can pass the --dry-run flag to your deployer of choice, but output is interleaved with skaffold logging (see #1937 (comment))

[tejal29]

This is dupe of #1739