Key & Host group including all keys/hosts

[pacoorozco]

**Is your feature request related to a problem?
If you want to use a group that includes ALL keys or hosts, you need to update it everytime you create a new key/host.

Describe the solution you'd like
Two special groups All keys and All hosts that allow you to define all existent keys/hosts in every moment. It would be used only in policies. The name of this group should not be allowed as group name.

Describe alternatives you've considered
Creating an auto-updated group, but it's better to deal with this only in policies.