Add 'log-format' CLI flag, along with associated config flag, for 'vault server' command.

[mjarmy]

Add a log-format CLI flag that can specify either "standard" or "json" for the log format for the vault server command. Also add a log_format config file flag that does the same thing. If the format is not specified, then standard log format is used.

We already had the ability to specify this via an environment variable. Note that unlike some of the other environment variable flags, the definition of how json format is enabled is rather permissive, since it allows two different names for the variable, and a few different names for json format. This has been preserved for backwards compatibility. See https://github.com/hashicorp/vault/blob/issue-6796/sdk/helper/logging/logging.go#L67-L81

The precedence of the flags for this setting is CLI, then env var, then config file.

This pull request address issue #6796

[hashicorp-cla]

All committers have signed the CLA.

[catsby]

It looks like ParseEnvLogFormat is untested, do you think it's necessary to test it? I only see it used (in these files at least) when called from NewVaultLoggerWithWriter (which is also untested, but as a simple New* function I'm not worried about it)

[tyrannosaurus-becks]

This is looking really good, just a couple of minor questions.

[mjarmy]

@catsby It was a good idea to add a test for ParseEnvLogFormat(), so I did that

[tyrannosaurus-becks]

This looks great! I tested adding log_format = json to my config file, and I also tried the env var and CLI parameter. I also tried configuring various empty strings, and they defaulted to standard, and I tried doing it without any designation. Everything worked as expected. Good work!