[osutils/freebsd.py] param invite_group should be optional in create_user

[iblislin]

Weird logic...
I can not recall why i wrote this...

FreeBSDUtils().create_user('bsdtest', 'pw')

/root/bsd-cloudinit/cloudbaseinit/osutils/freebsd.py in create_user(self, username, password, invite_group, password_expires)
     29 
     30         assert not invite_group or isinstance(invite_group, list), "param invite_group must be a list."
---> 31         assert invite_group, "invite_group cannot be empty."
     32         for i in invite_group:
     33             grouplist += i+','

AssertionError: invite_group cannot be empty.