Fix Migrations.all_migrations() list by clearing cache

Creating Tenant using TenantMixin fails. TenantMixin.create_schema use migrate_schemas command. Command doesn't clear Migrations cache so settings.SOUTH_MIGRATION_MODULES filled with 'ignore' values is ingnored by south Migrations. This cause use on tenant schema migrations also from shared schema.
Fix just clear south Migrations cache after changing settings.SOUTH_MIGRATION_MODULES.
Probably it can be done better, but this solve issue, and I think that place is intuitive.