Fix up Cluster nodeEvac unittests for shared storage

Currently, these presume that the instance being generated always is
DRBD/relocatable (but won't be in the future), and also presume that
all relocation modes can be used (hint: not ☺️

).

So we add a couple of helper functions to restrict the used disk
templates/relocation modes, and these two tests are ready for shared
storage.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>