engine: quota caching fix

reverts the memory and vcpu usage to it's original value when the quota
request is rejected

Change-Id: I0fad37c170e27d5cb78445c7c2fbcd9b3c7f9116
Signed-off-by: Laszlo Hornyak <lhornyak@redhat.com>
Bug-Url: https://bugzilla.redhat.com/894869