Nailgun/fuel-agent integration

Added provision_metadata JSON field into release model.
This field is necessary to make nailgun able to provide
fuel-agent with the information about which image to download,
what it's format, etc. This field also contains information
about which provisioning method to use (cobbler or image).

Change-Id: I0f15b0e713d7ec97e67d26f4eba3cd76bdc18806
Implements: blueprint image-based-provisioning