support for tunnel interfaces (wireguard) in network configuration

[defect-track]

Adds support for Wireguard using systemd-networkd configuration files.

Closes #664

[stgraber]

@defect-track could you edit your commit message to include the Signed-off-by line?
There are also a few static analysis failures that should be looked at :)

[defect-track]

@stgraber updated the code based on reported errors. For the documentation error we properly need to add WireGuard as this is an unknown word for the spellchecker as it looks. Added the --signoff option to the latest commit.

Will wait on the next run to see if there are more changes required.

[stgraber]

I'm waiting for #724 to clear, then I'll look into this one, will rebase, make any small tweak needed and send it to get merged, thanks!

[stgraber]

I've rebased the branch, merged the two commits together and addressed the review comments from @gibmat.

I still need to do a review myself, split the commits a bit, but we should at least have it at a stage now where the tests should all be passing.