Schema enhancement to support MultiSegment Network

Schema enhancement to support MultiSegment Network

Description:
Currently, there is nothing in the schema that ensures segments
for a network are returned in the same order they were specified
when the network was created, or even in a deterministic order.

Solution:
We need to add another field named 'segment_index' in
'ml2_network_segment' table containing a numeric position index.
With segment_index field we can retrieve the segments in the
order in which user created.

This patch set also fixes ML2 invalid unit test case in
test_create_network_multiprovider().

Closes-Bug: #1224978
Closes-Bug: #1377346

Change-Id: I560c34c6fe1c5425469ccdf9b8b4905c123d496d