core: organize RemoveDiskCommand#canRemoveTemplateDisk

1. Return false as soon as we detect condition that is not met, instead
of using the 'retValue' variable
2. Extract some code to seperate method in order to make this method
more concise and readable

Change-Id: I7ddc1e6abd85c61ac1486cdd60744124ebfb27b9
Signed-off-by: Arik Hadas <ahadas@redhat.com>