CLI: subcommand for saving configuration

[ava-affine]

unitctl should have an option to save the current running configuration, including NJS configuration and certificates.

Currently the closest analogue is for users to run unitctl edit and then to save the file, but this only works in interactive environments and even then does not include NJS configuration or certificates. Its also a weird path to take when all you want to do is save your configuration.

Consider the following:
$ unitctl save golden-working-config.tar

Additionally, config saved with this subcommand should be immediately importable with unitctl import:

$ tar -xvf golden-working-config.tar
$ unitctl import golden-working-config

[avahahn]

A couple user stories that this work pertains to:

• As a support person, devops engineer, or sysadmin responsible for applications deployed in one or more instances of NGINX Unit, I want to be able to save a backup of a perfectly working configuration.
• As an app developer or support person working on deploying an application with sufficiently complex configuration requirements under NGINX Unit, I want to be able to easily extract an existing configuration that I can work on.
• As an application developer I want to be able to save and distribute a working application server configuration along with my application.

Per @lcrilly's comments in #1321 I think it is worth using unitctl export for the subcommand here.

[ac000]

Can this be closed?

[avahahn]

Not yet! We need to at least add support for js_modules.
I would be in support of editing this ticket, closing it, and then making a new one for the remaining work if that is preferrable.