Improved integration tests

This merge includes a refactored integration test framework, to better
take advantage of asyncio, and clarifies the test logic so it's easier
to extend in the future. It also supports testing of more complex VNFs,
such as multi-vdu, multi-charm VNFs.

n2vc/vnf.py:
- Remove duplicate status filtering, as it was a poor solution and lead
to situations where some callbacks were not sent.
- Added an internal refcount to track libjuju instantiation (helpful in
debugging)

tests/charms/layers/proxy-ci/reactive/proxy_ci.py:
- Fix name of install function (remove confusion while reading logs)

tests/base.py:
- Add debug() for more consistent and useful logging
- Refactor: remove parse_metrics
- Improved error handling
- Better LXD network connectivity verification
- Refactor test framework:
    - Better use of async coded
    - Make handling of test phase more robust
    - Support more complex test cases, such as multi-vdu, multi-charm

tests/integration/*
- Update to use refactored test framework

Other:
- `make clean` now removes charm artifacts in tests/charms/builds/
- `make lint` now ignores charm artifacts

Signed-off-by: Adam Israel <adam.israel@canonical.com>
Change-Id: I43a6d573a7bafdfe3ccb0bb0f0f7d75dcc9c42b1