[#1725] Move sort validation to datastore_validate()
Revert "[#1725] Datastore must be the first IDatastore plugin loaded"

This reverts commit df8c1af5fbdc436ed63c76a6f3470c8eae1c2b37.

Trying to figure out why the tests are failing.

Conflicts:
	ckanext/datastore/tests/test_plugin.py