This patch fixes an issue while performing share network update.
Manila was allowing the user to update a share network even if it
did not contain a default subnet. Now, we throw an error if there
is no default subnet to be updated. Also, adds an extra validation.
Now, Manila do not allow the default share network subnet update if
it is going to have only `neutron_net_id` or `neutron_subnet_id`
after the update.
Change-Id: Iba2761db53da68a1c497e2e5dd370c36a22ebbed
Closes-Bug: #1846836