This commit fixes a test that only fails on certain users machines where
Vagrant ends up trying to run real guest capabilities to test if the
docker provisioner raises an error if the provisioner install failed. It
fixes it by mocking out the expected return values for those
capabilities rather than relying on them actually running for this
specific unit test.
Prior to this commit, if a user attempted to configure
`/etc/default/docker` through vagrant prior to installation, the package
manager would not override an existing configuration and installing
docker would then fail. This commit fixes this by introducing a
`post_install_provisioner` that allows users to define a provisioner
block that will run after docker has been installed, allowing users to
configure `/etc/default/docker` how they want.
