Add LXD datasource (#1040)

Add DataSourceLXD which knows how to talk to the dev-lxd socket to
obtain all instance metadata API:
https://linuxcontainers.org/lxd/docs/master/dev-lxd.

This first branch is to deliver feature parity with the existing
NoCloud datasource which is currently used to intialize LXC instances
on first boot.

Introduce a SocketConnectionPool and LXDSocketAdapter to support
performing HTTP GETs on the following routes which are surfaced by the
LXD host to all containers:
http://unix.socket/1.0/meta-data
http://unix.socket/1.0/config/user.user-data
http://unix.socket/1.0/config/user.network-config
http://unix.socket/1.0/config/user.vendor-data
These 4 routes minimally replace the static content provided in the
following nocloud-net seed files:
/var/lib/cloud/nocloud-net/{meta-data,vendor-data,user-data,network-config}

The intent of this commit is to set a foundation for LXD socket
communication that will allow us to build network hot-plug features
by eventually consuming LXD's websocket upgrade route 1.0/events to
react to network, meta-data and user-data config changes over time.

In the event that no custom network-config is provided, default to the
same network-config definition provided by LXD to the NoCloud
network-config seed file.

Supplemental features above NoCloud datasource:
surface all custom instance data config keys via cloud-init query ds
which aids in discoverability of features/tags/labels as well as
conditional #cloud-config jinja templates operations based on custom
config options.
TBD: better cloud-init query support for dot-delimited keys

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