Added a new command line parameter (-i or --image=) that allows rbd-fuse to specify a single image to be made available within
the mount directory. The purpose of this is to allow a single RBD to be "mounted" in userspace without opening (and locking)
the other RBDs in the pool.

This is accomplished by performing a case-sensitive string compare in enumerate_images() when an image name is
specified on the command line. If no image name is specified, all images appear in the mount directory. If a non-existent
image name is specified, the mount directory is empty.

Signed-off-by: Stephen F Taylor <steveftaylor@gmail.com>
Changed the -i parameter to -r in order to avoid a conflict with a generic flag interpreted by the common code.
Merge pull request #1737 from steveftaylor/add_rbd_fuse_image_restriction

Added a new command line parameter (-i or --image=) that allows rbd-fuse...

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>