Fix auto-deletion of ports when deleting subnets in ML2

When a subnet is deleted, certain ports referencing it are
auto-deleted. The implementation of NeutronDBPluginV2.delete_subnet()
does this at the DB level, so ML2's mechanism drivers were not being
called.

Ml2Plugin.delete_subnet() is changed to not use the base class's
method, and to auto-delete ports by calling its own delete_port()
method outside of the transaction. A loop avoids race conditions with
ports being asynchronously added to the subnet.

The logic in Ml2Plugin.delete_network() is also fixed to properly
handle auto-deleting ports and subnets, and debug logging is added to
the various delete methods.

Closes-Bug: 1234195
Partial-Bug: 1235486
Change-Id: I6d74f89d39ea8afe6915f1d2f9afdf66c0076f5a
Imported Translations from Transifex

Change-Id: Iadb02e0009605ba739d6c7e6ccbe1e62331f94fe
Fix migration

In migration 338d7508968c_vpnaas_peer_address_ was mistake in
usage alter_column's parameter type_.

Closes-Bug: #1238561

Change-Id: I6e0eb8260cf5e8d8f557b174c7985e2f7ca1c912
Merge "Fix migration"
Merge "Fix auto-deletion of ports when deleting subnets in ML2"