Automate create composite primary keys

One important pre-requisite for migration to citus is for all the models which are inherited from TenantModel, we need to replace the primary key on id column to a composite primary key (tenant_id,id). As we did this for foreign keys in this #16 (specifically in this [file](https://github.com/citusdata/django-multitenant/pull/16/files#diff-cea67ff3eab64345dcc7b719b50bebd7)), we could do automate dropping single primary keys and creating composite primary keys.