Support environment variable configuration

[jessepeterson]

Currently we just use CLI flag parsing. Support configuring via environment variable fallback.

[steviec]

@jessepeterson, I'll take this on, it would be helpful for my setup. Should I follow the same pattern I see in mysqlscepserver using envString() and envBool()?

[sduc]

@jessepeterson, It would be useful for our usecase to have the cli flags as env variables, like it is for example done on the SCEP server. Happy to take this if this is still something you guys are happy to have.

[jessepeterson]

I'm not opposed @sduc! I'd want to keep it extremely lightweight, keeping the stdlib flag package (such as the simple environment readers that @steviec suggested, with some improvements). There's also a wrinkle with multiple-use switches, in particular the -storage and related functions which have a custom flag accumulator:

nanomdm/cmd/nanomdm/main.go

Lines 56 to 60 in a192ac3

	cliStorage := cli.NewStorage()
	flag.Var(&cliStorage.Storage, "storage", "name of storage backend")
	flag.Var(&cliStorage.DSN, "storage-dsn", "data source name (e.g. connection string or path)")
	flag.Var(&cliStorage.DSN, "dsn", "data source name; deprecated: use -storage-dsn")
	flag.Var(&cliStorage.Options, "storage-options", "storage backend options")