Add the OVA container format

An OVA package is a tar archive usually containing an OVF directory
inside it. Nova needs to be able to differentiate OVF and OVA based on
the container format in order to extract the relevant information from
it.
This patch adds the OVA container format to the Glance configuration.

Closes-Bug: #1286375

Change-Id: I5034d2943907823a9296e7a5fb41a28f2d92ec5a
Merge "Add the OVA container format"