Fix unnecessary snap of glance image, with non-raw images

When the glance image format is not raw, the glance image stored on
GPFS should not be snapped. Because, we copy the image to cinder volume
repository converting the image format to 'raw'. So, no copy_on_write
is supported.

Change-Id: I9cc3ffb2f36b65876b05ecb9f4654461be4516b7
Closes-Bug: #1378964
Merge "Fix unnecessary snap of glance image, with non-raw images"