Add a -f option to let a user pass a filename for creating secrets

[cyli]

#1589 This would read the secret from the file instead of STDIN.

This PR also servers as a platform for continuing the discussion from #1567 (comment) and doing a design discussion of what docker secret create <secret-name> should look like.

A summary of that discussion and some IRL discussions:

@aluzzardi suggests the following UI:

• Pass as an argument: docker secret create foo bar
• Pass to stdin: echo bar | docker secret create foo -
• Pass as a file: echo bar > value.txt && docker secret create foo -f value.txt

The reason the current design does not to accept the secret itself as an argument is that we do not want to encourage users to type the secret into a command. If we did accept a secret as a direct argument, then it would be available in the shell history of any user that created one, unless they explicitly removed it.

With STDIN and file, we can't prevent them from echoing to STDIN or a file, but it doesn't legitimize that as a way to add a secret.

cc @dmp42 on UI
cc @diogomonica

[codecov-io]

Current coverage is 53.89% (diff: 100%)

Merging #1595 into master will increase coverage by 0.06%

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+ Misses         5429       5420     -9   
+ Partials       1009       1006     -3   

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[diogomonica]

Don't have preferences here as long as we chose our examples in the docs wisely.

[aaronlehmann]

Looks fine to me. I'm sure the real debate will play out when the Docker PR is reviewed, though.