allow to specify CLI options via environment vars

Use `git show -w` to inspect this commit diff since
it changes whitespace due to `gofmt`.

This commit allows to use environment variables in place of
CLI parameters to change program behavior. This supports the
notion of [12-factor apps](http://12factor.net/)
and makes it easier to dockerize the app as well as create
a systemd unit file.

It establishes a pattern of `MGMT_*` naming convention
for environment variables.

main.go

@@ -228,6 +228,7 @@ func main() {
 					Name:   "file, f",
 					Value:  "",
 					Usage:  "graph definition to run",
+					EnvVar: "MGMT_FILE",
 				},
 				cli.BoolFlag{
 					Name:  "no-watch",
@@ -259,16 +260,19 @@ func main() {
 					Name:   "seed, s",
 					Value:  "",
 					Usage:  "default etc peer endpoint",
+					EnvVar: "MGMT_SEED_ENDPOINT",
 				},
 				cli.IntFlag{
 					Name:   "converged-timeout, t",
 					Value:  -1,
 					Usage:  "exit after approximately this many seconds in a converged state",
+					EnvVar: "MGMT_CONVERGED_TIMEOUT",
 				},
 				cli.IntFlag{
 					Name:   "max-runtime",
 					Value:  0,
 					Usage:  "exit after a maximum of approximately this many seconds",
+					EnvVar: "MGMT_MAX_RUNTIME",
 				},
 			},
 		},