feat(yaml): Support document references

This allows the user to apply and/or validate a manifest using
either a filepath (as before) or URL.

Addition by sh8121att:

- Create a general document resolver class to handle local paths
  and URIs
- Allow multiple filenames and combine them into a single document
  set
- Change API to allow for passing document reference URIs to be
  resolved server-side rather
- Update validation API to conform to UCP specification
- Dockerfile updates to speed up build
- Fix unit tests

Closes #96

Change-Id: I5a57779f10d1b63ffc161a14afec851a34ae9efe

tox.ini

@@ -40,6 +40,9 @@ commands =
   bandit -r armada -x armada/tests -n 5
 
 [testenv:coverage]
+passenv=http_proxy https_proxy no_proxy HTTP_PROXY HTTPS_PROXY NO_PROXY
+setenv=
+    VIRTUAL_ENV={envdir}
 commands =
   python -m pytest \
     --cov-branch \