Improvements around hibernation

[ialidzhikov]

1. g info currently only shows number of Shoots per Seed. It is frequently used it to check how many Shoots are hosted on the Seed. Along with the total count, a good improvement would be to add the count of currently hibernated and running Shoot control planes to get a better overview.

$ g info
Garden: foo
Shoots:
  total: 5
  unscheduled: 0
  aws-us3:
    active: 3
    hibernated: 2

2. g shell could be hibernation aware. Currently it shows an EOF error:

$ g target shoot hibernated-one
$ g shell
Error: Get $HOST/api/v1/nodes: EOF

It could show a better error message such as:

$ g shell
Error: shoot "hibernated-one" is hibernated

Definition of done

• 1. g info to show hibernated Shoots.
• 2. g shell to handle properly hibernated Shoots