flavors: Re-visit and add description of each

[obedmr]

In order to have a better UX and provide details on each flavor, we need to show details on the default list of supported flavors.

Let's do something like OpenStack's flavors

Flavor	VCPUs	Disk (in GB)	RAM (in MB)
tiny	1	1	512
small	1	20	2048
medium	2	40	4096
large	4	80	9192
xlarge	8	160	16384

It will require changes on following packages:

• vm
• pkg/cli