Update provision_slurm.py.j2 to import defaultdict to silence this error

Traceback (most recent call last):
 File "/usr/local/sbin/provision_slurm.py", line 100, in <module>
   mk_slurmuser_batch(usergroup,"default")
 File "/usr/local/sbin/provision_slurm.py", line 67, in mk_slurmuser_batch
   userdict = defaultdict(list)
NameError: global name 'defaultdict' is not defined