Tests should be protected against behavioural changes of dependencies

[jodh-intel]

PR #165 ended up failing at the Travis stage since .travis.yml does this:

git clone https://github.com/containernetworking/cni.git

Unfortunately, CNI commit 13824487c6f97a8ffde7b4ef10330c75ddc34826 changed the behaviour of the CNI package such that the virtcontainers test TestRemoveNetworkFailureNetworkDoesNotExist failed. That test has been reworked on #167, but we need to find a way to avoid this sort of breakage in future.

Thoughts:

• add cni as a vendor/ package to virtcontainer.
• change .travis.yml so that it checks out the "last known good commit of the CNI package.