Add getting and setting snap config

[st3v3nmw]

TL;DR

This PR:

• Adds functionality to set the snap config (via SnapManager) & periodically syncing the config (via SnapMonitor)
• Migrates Snap HTTP functionality to use the snap-http package

---

Deets

This started off #202 (conditionally use snap_http, fall back to local module) but ran into some issues while using both snap-http & the local landscape.client.snap simultaneously:

• For HTTP responses, landscape.client.snap returns a dict while snap_http returns SnapdResponse
• snap_http's SnapException is missing the json method in landscape.client.snap's SnapdHttpException

Applied the git submodule suggestion in #202 and created a symbolic link: landscape.client.snap_http -> snap-http/snap_http and deleted landscape.client.snap.

--

Other noteworthy change: Added a new "schema type" Nested that allows nesting of "container types" like List, Dict, & KeyDict. The values of snap configs have arbitrary types and can be deeply nested. Consider values like:

# A
{"bar": "default", "baz": False}

# B
{
            "foo": {"baz": "default", "qux": [1, True, 2.0]},
            "bar": "enabled",
}

# C
{
            "foo": {"bar": "baz", "qux": {"quux": "corge"}},
            "ssids": [{"name": "Airport Wi-Fi"}, {"name": "Totally Legit Wi-Fi"}],
}

Arbitrary values cannot be generally/succinctly represented with the existing "schema types" esp. very very nested values. The only other option is using a type like Unknown that doesn't do any schema validation.

[Perfect5th]

Arbitrary values cannot be generally/succinctly represented with the existing "schema types" esp. very very nested values. The only other option is using a type like Unknown that doesn't do any schema validation.

Because the shape of this data is essentially arbitrary, and therefore cannot cleanly map to anything on the Landscape Server side, database-schema-wise, I'm going to suggest that we just use Unicode as the type, and serialize it as JSON. Is it guaranteed to be serializable?

There's some precedence for doing it this way.

[st3v3nmw]

Is it guaranteed to be serializable?

Yes, the GET /v2/snaps/{name}/conf endpoint returns a JSON map of configuration keys and values. Let me switch to Unicode

[Perfect5th]

LGTM, thanks!