Tox + Integration testing

This commit implements a VNF Descriptor-driven integration
test framework, which will lead to integration tests being able
to run via jenkins, and more robust testing in general.

N2VC:

- Allow the use of an event loop passed when instantiating N2VC
- Refactor the execution of the initial-config-primitive so that it can
be easily re-run, such as the case of when a proxy charm is deployed
before the VNF's VM is created.
- Refactor GetPrimitiveStatus, to return the status (queued, running,
complete, failed) of a primitive.
- Add GetPrimitiveOutput, to return the output of a completed primitive
- Fix model disconnection when executing a primitive (it was happening
in the wrong scope)
- Fix wait_for_application, which was previously unused and broken
- Add support for parameter's 'data-type' field
- Add support for better SSH key management, allowing for a proxy charm
to be deployed before the VNF, so that it's public SSH key can be
injected when the VNF's VM is created.

Integration Tests:

The integration tests are intended to exercise the expected
functionality of a VNF/charm: deploy the charm, configure it as required
(i.e., ssh credentials), and execute the VNF's
initial-config-primitives.

- test_native_charm: deploy a native charm to a juju-managed machine and
verify primitive execution works
- test_proxy_charm: deploy a proxy charm, configured to talk to a remote
machine, and verify primitive execution works
- test_metrics_native: deploy a native charm and collect a metric
- test_metrics_proxy: deploy a proxy charm and collect a metric from the
vnf
- test_no_initial-config-primitive: deploy a vnf without an
initial-config-primitive
- test_non-string_parameter: deploy a vnf with a non-string parameter in
initial-config-primitive
- test_no_parameter: deploy a vnf with a primitive with no parameters

General:
- Add a build target to tox.ini so that a .deb is built via Jenkins

TODO (in a follow-up commit):
- test multi-vdu, multi-charm
- test deploying a native charm to a manually-provisioned machine
- Update inline pydoc
- Add more integration tests
- Add global per-test timeout to catch stalled tests

Signed-off-by: Adam Israel <adam.israel@canonical.com>
Change-Id: Id322b45d65c44714e8051fc5764f8c20b76d846c