Convert all incoming protocol numbers to string

PostgreSQL is more sensitive for types than MySQL when it selects
something from columns in database. So it fails when it tries to
select from string field comparing with integer value.

Added unit test to verify conversion of protocol numbers to
strings.

Closes-bug:#1381379

(cherry picked from commit bc34f26302faea116be7e61051c32f8787530836)

Conflicts:
	neutron/tests/unit/test_extension_security_group.py

Change-Id: I0a29595403a07c66888871088d5549705a097f68
(cherry picked from commit c00b4b31276f9ae21fa0a7a213391277bda4cfa7)