[RFC] Support multiple LXD servers

[jtopjian]

I'm looking for feedback on how to best implement this.

I can see a few ways of implementing this:

1. Allow all configuration to be repeatable:

[servers.default]

# ...

  [servers.default.discovery]
  kind = "lxd"

  [servers.default.discovery.lxd.local]
  container_label_key = "user.gobetween.label"
  container_port_key = "user.gobetween.port"
  container_label_value = "web"
  generate_client_certs = true
  accept_server_cert = true
  server_address = "unix:///var/lib/lxd/unix.socket"


  [servers.default.discovery.lxd_servers.lxd-02]
  container_label_key = "user.gobetween.label"
  container_port_key = "user.gobetween.port"
  container_label_value = "web"
  generate_client_certs = true
  accept_server_cert = true
  server_address = "https://lxd-02.example.com:8443"
  server_remote_password = "password"

2. Have one global section and a section for each lxd server. The global settings would be applied to all servers. Each lxd server entry would only have a few configuration settings.

[servers.default]

# ...

  [servers.default.discovery]
  kind = "lxd"

  [servers.default.discovery.lxd]
  container_label_key = "user.gobetween.label"
  container_port_key = "user.gobetween.port"
  container_label_value = "web" 
  generate_client_certs = true
  accept_server_cert = true

  [servers.default.discovery.lxd_servers.local]
  server_address = "unix:///var/lib/lxd/unix.socket"

  [servers.default.discovery.lxd_servers.lxd-02]
  server_address = "https://lxd-02.example.com:8443"
  server_remote_password = "password"

3. Similar to 2, but the global settings are the standard settings in the [servers.default.discovery] namespace:

[servers.default]

# ...

  [servers.default.discovery]
  kind = "lxd"
  lxd_container_label_key = "user.gobetween.label"
  lxd_container_port_key = "user.gobetween.port"
  lxd_container_label_value = "web"   # (required) Label to filter containers
  lxd_generate_client_certs = true
  lxd_accept_server_cert = true

  [servers.default.discovery.lxd_servers.local]
  server_address = "unix:///var/lib/lxd/unix.socket"

  [servers.default.discovery.lxd_servers.lxd-02]
  server_address = "https://lxd-02.example.com:8443"
  server_remote_password = "password"

4. Allow all settings to be global and per server. If a setting does not exist in a server, it takes the global value.

This draft PR implements 3 because it seemed like a good mix of everything to critique.

The following configuration settings are able to be set in a per-server config:

• ServerAddress
• ServerRemotePassword
• ContainerInterface
• ContainerAddressType

All other settings are global, meaning their values are applied to all servers.

There's no rush to review this :)