core: Verifying storage space for AddDiskCommand

This is the first in a series of patches intended to fix storage space
allocation validation throught the system (see bz).
Added hasSpaceForNewDisk(s) in StorageDomainValidator. Added test -
StorageDomainValidatorFreeSpaceTest.
Applied use in AddDiskCommand (former use is buggy).

Amended AddDiskToVmCommandTest. Previous tests simulated several space
situations. These are now thoroughly tested in
StorageDomainValidatorFreeSpaceTest, where they belong, and therefore
removed from the command test. What's left to
test is that the command actually behaves according to the validator's
response. To that end added two new tests.

Also amended AddDiskCommand CDA to always check for threshold limits
(not only preallocated) since we want to have this leeway.

Bug-url: https://bugzilla.redhat.com/960934
Change-Id: I1a33502683ec77fba09efffba1438beb552082f7
Signed-off-by: Vered Volansky <vvolansk@redhat.com>