webadmin: Template's empty string as a disk alias UI fix.

While making a new template based on a VM, providing the template's
allocated disks's alias as an empty string is now disabled ALSO BY UI.
the user should not be able to create a template with disks that have
an empty aliases- this bug has already been fixed at the BACKEND, this
current patch prevents the user from giving an empty alias at UI level.

Change-Id: I246063773e423f1e34ace3acd3f3884b282597e7
Bug-Url: https://bugzilla.redhat.com/1110304
Signed-off-by: Amit Aviram <aaviram@redhat.com>