RBD: use image_conversion_dir for image operations

This patch deprecates the rbd-only 'volume_tmp_dir' from the rbd driver
in favor of image_conversion_dir for image operations.

Closes-Bug: #1375490
DocImpact: config option removal
Change-Id: I46ab21c2a90be136d1eb677fcefcc3d65c48a3c5
Merge "RBD: use image_conversion_dir for image operations"