Initial client implementation (rhyzomectl)

I took a stab at a CLI client. It has lots to be desired but will help me write fewer crazy curl commands.

Some sample usage:

$ rhyzomectl         
Rhyzome is a free/libre cloud computing platform, aimed at being less
		shitty than the others. So far it has not succeeded.

		rhyzomectl can be used to interact with rhyzome servers. Wouldn't it be nice
		if there was some documentation here? When you figure out whatever it is you
		came here to figure out, maybe you should update this string!

Usage:
  rhyzomectl [command]

Available Commands:
  instance    Commands to access instances

Flags:
  -h, --help            help for rhyzomectl
  -s, --server string   The remote rhyzome host to use

Use "rhyzomectl [command] --help" for more information about a command.

$ rhyzomectl help instance
...

Usage:
  rhyzomectl instance [flags]
  rhyzomectl instance [command]

Available Commands:
  create      Create a new instance
  list        List all instances on this server

Global Flags:
  -s, --server string   The remote rhyzome host to use

Use "rhyzomectl instance [command] --help" for more information about a command.

$ rhyzomectl -s http://10.5.1.232:8080 instance 
radios
nvr
monitoring
ubnt
[...]

$ rhyzomectl --server http://10.5.1.232:8080 instance create --name sample-instance --base-image debian --disk 25
2020/05/03 01:30:38 started
2020/05/03 01:30:38 Connecting to libvirt
2020/05/03 01:30:39 Creating volume
2020/05/03 01:30:39 Downloading base image
2020/05/03 01:34:25 Creating domain
2020/05/03 01:34:25 Booting domain
2020/05/03 01:34:27 Domain booted
2020/05/03 01:34:27 completed