Indicate supported instance types when querying /1.0

[MusicDin]

Add ServerEnvironment field for instance_type indicating which instance types are supported by the server.

node-1:~# lxc query /1.0 | jq '.environment | {server_name, instance_types}'
{
  "server_name": "node-1",
  "instance_types": [
    "virtual-machine",
    "container"
  ]
}


node-1:~# lxc query /1.0?target=node-2 | jq '.environment | {server_name, instance_types}'
{
  "server_name": "node-2",
  "instance_types": [
    "virtual-machine",
    "container"
  ]
}

c2:~# lxc query /1.0 | jq '.environment | {server_name, instance_types}'
{
  "server_name": "c2",
  "instance_types": [
    "container"
  ]
}

API extension is named server_instance_type_info.

[tomponline]

@MusicDin please can you add a test to check for the container instance type support? Thanks

[tomponline]

LGTM thanks!

[tomponline]

@MusicDin needs a rebase please

[tomponline]

@MusicDin needs a rebase again please