Fix gnt-node evacuate w. iallocator

Commit 2bb5c911 moved around and changed the _RunAllocator function in
the DiskReplace → TaskLet conversion, but in the process it changed the
relocate_from argument from a list of nodes to just the secondary node.
This breaks the protocol and current iallocator scripts.

This patch fixes that but also adds a local variable 'instance' since
it's not nice to write self.instance so many times.

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