Check shell command receives enough arguments

Summary:
shell allows handlers with an arbitray number of required arguments.

This change handles the case where such an argument is missing.

Test Plan:

handlers:
  tools:
    server: "dev.tools.wmflabs.org"
    command: ['become', '{{%s}}']
    interactive: True

$ shell tools
shell: Required argument is missing.
$ shell tools translate
tools.translate@tools-bastion-02:~$

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1496