Change internal API for the IAllocator class

Currently the 'name' parameter in the constructor is required (as a
non-keyword argument). Since the (to follow) node evac IAllocator mode
doesn't have 'name' as a valid argument, we're moving this one into the
per-request key, leaving the constructor required arguments more
abstract.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>