Basic import conversion of image format

With the support of asynchronous operations and the new import workflow in Glance (see the dependency tree), it will possible to have an image format conversion on the fly while importing an image.

The conversion will be provided by a plugin of the import workflow. This plugin can be activated
or not based on the deployer configuration. This means that the deployer will need to specify the
preferred format of images for the deployment.

This blueprint will handle the conversion of formats supported by qemu-img convert: raw, qcow2, vdi, vmdk and vpc.

At this point this is not clear if we need one plugin per conversion or one conversion plugin.

Internally, Glance will receive the bits of the image in a XX format. These bits will be stored in a temporary location. The plugin will be triggered to convert the image to its target format YY and moved to its final destination. When the task is finished, the temporary location is deleted.
This means that the format uploaded initially is not kept by Glance.