addons: link: porting 'veth-peer-name' attribute to python3 branch

addons: link: porting 'veth-peer-name' attribute to python3 branch

original commit ported to the python3 branch:

commit bffa619b11ae7aa9e567c26c255c17ac6df2c495
Author: Maximilian Wilhelm <max@rfc2324.org>
Date:   Sat Jan 14 19:08:01 2017 +0100

    Add option 'veth-peer-name' to veth links and ensure proper configuration.

      The option »veth-peer-name« forces an veth peer link to be created with
      a specific interface name. As the interface name of the "local" part of
      the veth link pair already is defined by the name of the interface stanza
      this option is added to, now both sides are clearly named.

      As there is a bidirectional dependency of both link pairs - both cannot
      exist without the other - this presents a problem when setting up all
      interfaces. Depending on which interface is set up first there might be
      a problem when only on dependency is specified. Therefore adding the
      »veth-peer-name« option to both interface of the veth link pair ensures
      that regardless of which side is configured first the peer name will be
      set correctly. This intentionally creates a circular dependency which is
      handled accordingly.

      Fixing the config check for veth link-type while at it :)

    Signed-off-by: Maximilian Wilhelm <max@rfc2324.org>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>