Make "command" parameter as positional

[hongbin]

At "run" and "create" command, we use "--command" to take a command. For example:

  $ zun run --command "sleep 10000" cirros

I think it is better to change the syntax as:

  $ zun run cirros sleep 1000

The goals of this change is:
* Make it consistent with "zun exec" (zun exec <container> sleep 1000)
* Make it consistent with docker cli (docker run -d cirros sleep 1000)

NOTE: this change is significant so requires a team discussion first.