Create a zuul_data fixture for testinfra.
The fixture directly loads the inventory from the inventory YAML file
written out. This lets you get easy access to the IP addresses of the
hosts.
We pass in the "zuul" variable by writing it out to a YAML file on
disk, and then passing an environment variable to this. This is
useful for things like determining which job is running. Additional
arbitrary data could be added to this if required.
Change-Id: I8adb7601f7eec6d48509f8f1a42840beca70120c
0d004ea73d