Add firewall rule management to `cb network` command.

[abrightwell]

These changes move firewall rule management under cb network and deprecate the cb firewall rule command. This comes as part of a request to be able to update firewall rules more completely as described in #161.

The following commands are now supported:

• cb network add-firewall-rule
• cb network list-firewall-rules
• cb network remove-firewall-rule
• cb network update-firewall-rule

Each command requires the specification of the network to peform the operation. As well, the shell completion logic has been updated to suggest the list of available networks and firewall rule when applicable.