Fix incorrect change of Enum type

Fix incorrect change of Enum type

In migration 1341ed32cc1e_nvp_netbinding_update Enum type had been
changed incorrectly from ('flat', 'vlan', 'stt', 'gre')
to ('flat', 'vlan', 'stt', 'gre', 'l3_ext') for PostgeSQL.

The same problem is taken place for vlan_type in migrations
38fc1f6789f8_cisco_n1kv_overlay from ('vlan', 'vxlan', 'trunk',
'multi-segment') to ('vlan', 'overlay', 'trunk', 'multi-segment')
and in 46a0efbd8f0_cisco_n1kv_multisegm from ('vlan', 'vxlan') to
('vlan', 'vxlan', 'trunk', 'multi-segment').

In this change request was added separate method for changing Enum
type for PostgreSQL.

Closes-bug: #1301396

Change-Id: I27197fb7405630a55178be8516a4b62bd135e05c