net: exclude OVS internal interfaces in get_interfaces

[OddBloke]

Proposed Commit Message

net: exclude OVS internal interfaces in get_interfaces

`get_interfaces` is used to in two ways, broadly: firstly, to determine
the available interfaces when converting cloud network configuration
formats to cloud-init's network configuration formats; and, secondly, to
ensure that any interfaces which are specified in network configuration
are (a) available, and (b) named correctly.  The first of these is
unaffected by this commit, as no clouds support Open vSwitch
configuration in their network configuration formats.

For the second, we check that MAC addresses of physical devices are
unique.  In some OVS configurations, there are OVS-created devices which
have duplicate MAC addresses, either with each other or with physical
devices.  As these interfaces are created by OVS, we can be confident
that (a) they will be available when appropriate, and (b) that OVS will
name them correctly.  As such, this commit excludes any OVS-internal
interfaces from the set of interfaces returned by `get_interfaces`.

LP: #1912844

Test Steps

Use included integration test.

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[OddBloke]

Looking at this again, I wonder if this is going to mean that this OVS-enabled image is used for all future launches: I'll check now.

[OddBloke]

It is; working on this now.

[raharper]

This looks well done. Some in-line questions.

[TheRealFalcon]

I think main implementation and unit tests look good. I'll revisit when the WIP tag is removed since you're still working on the integration test.

Not necessary for this PR but...
Given the fairly random places I'm seeing

@mock.patch(
    "cloudinit.net.is_openvswitch_internal_interface",
    mock.Mock(return_value=False),
)

I'm wondering if it might make sense to create a top-level autorun fixture that provides default mocks for things that we obviously don't want running in unit tests. In this case we would default is_openvswitch_internal_interface to False, and then actual openvswitch tests could change it to True when needed.

[fnordahl]

I'm speaking only for the Open vSwitch specific bits in this patch set, and as we have discussed on the LP this is a good approach and LGTM.