Support cinder_img_volume_type in image metadata

Support cinder_img_volume_type in image metadata

This patch adds a feature to specify Cinder volume type
via cinder_img_volume_type parameter in glance image metadata.

In some cases, between image and hypervisor, storage backend
are tightly connected in each other. For example, if users
want to boot a VMware instance from an image, this image has
to be configured some VMware specific parameters in previous,
and then Nova can deploy an instance on to proper hypervisor
based on the image metadata properties.

Currently, Cinder handles few image metadata properties but
volume type is not included. If volume type can be retrieved
from image metadata which is configured by cloud admin, user
doesn't need to care volume type and also storage backends.
And then appropriate volume type will be chosen automatically
based on the information of cinder_img_volume_type in the
image metadata during volume creation.

If user has enough knowledge about image, hypervisor and
storage backend, user also can specify 'volume_type' via
CLI or API as in the past.
Priority of volume type related parameters are shown below.

1. volume_type             (via API or CLI)
2. cinder_img_volume_type  (via glance image metadata)
3. default_volume_type     (via cinder.conf)

DocImpact: Add usage of this parameter to 'Manage volumes'
           section in Cloud Administrator Guide
Change-Id: I62f02d817d84d3a7b651db36d7297299b1af2fe3