core: use Java naming convention

In CreateSnapshotCommand changed mnewCreatedDiskImage data member to
newDiskImage.
Also dropped the 'Created' for a shorter name, which is just as clear.

Change-Id: I614acc66ff631202b786f65daa2451803304ba4d
Signed-off-by: Vered Volansky <vvolansk@redhat.com>