Support file injection on container creation

Nova has the '--file' option on "openstack server create". This option can do file injection into the instance. Zun would like to implement a similar feature. In particular, we need:

1. An API for end-users to pass custom files into the server.
2. Under the hook, Zun will put these files into a temporary directory inside the compute host and mount it into the container.

Use case: containerized application like etcd (https://coreos.com/etcd/docs/latest/v2/docker_guide.html) requires to inject SSL certificates into the container.