Record and log reason for dhcp agent resync

A dhcp resync can be triggered at a number of points, but the actual
resync is done asynchronously by a helper thread.  This means by the
time the resync happens, it's hard to establish what actually caused
it.

There have been a number of problems in production systems that cause
excessive resyncs.  One is a ipv6/dnsmasq issue (rhbz#1077487) and
another is db corruption with duplicate entries [1].  The resync
triggers a whole lot of logs itself, so it becomes very unclear how to
establish any causality.

This change keeps track of what triggered the resync with some helpful
information.  We add a schedule_resync() function to to replace a
explicit set of needs_resync which tracks why it was called.

[1] The logs will contain output like "DBDuplicateEntry
(IntegrityError) (1062, "Duplicate entry
'6d799c6a-7a09-4c1e-bb63-7d30fd052c8a-d3e3ac5b-9962-428a-a9f8-6b2' for
key 'PRIMARY'") ..." in this case

Closes-Bug: #1294445
Change-Id: I9b1c6202f5a6bbad8589a8b64b2a38c9d9edb43b
Remove run-time version checking for openvswitch features

The current method of checking openvswitch and kernel versions for
specific feature support is brittle, distro-specific and unsupportable.

This patch removes the runtime version checks and implements a test
script which allows testing specific neutron features or can test
that all features required by a specific configuration are available.

For example, to test VXLAN support in openvswitch, either:

  neutron-sanity-check --ovs_vxlan

or pass in the deployed configuration files with AGENT/tunnel_types
containing 'vxlan', like:

  neutron-sanity-check --config-file /etc/neutron/plugins/ml2.conf.ini

Deployment tools can then test that the required features exist without
relying on version numbers and without incurring a penalty every time
the agent is started.

Implements: blueprint remove-openvswitch-version-check
DocImpact
Change-Id: I5e822d97b9bece9242ba5ac8901aa0ca4f090cfe
Allow neutron-sanity-check to check OVS patch port support

This patch allows to check OVS patch port support using:

  neutron-sanity-check --ovs_patch

or pass in the deployed configuration files with a non-empty
AGENT/tunnel_types (indeed patch ports are used to interconnect
br-int and br-tun):

  neutron-sanity-check --config-file ml2.conf.ini

DocImpact
Related-Bug: #1285335
Change-Id: Ic7bfabd1f4b85bfa490dd06c70eb5682ee42a681
Merge "Record and log reason for dhcp agent resync"
Merge "Remove run-time version checking for openvswitch features"
Merge "Allow neutron-sanity-check to check OVS patch port support"