Initial implementation of iol2 STP

[DanPartelly]

Based on Arista.

It passes all netlab integration tests and was also validated on a lot of topologies from:

https://github.com/ipspace/netlab-examples/tree/master/STP/examples

Doesn't handle stub port types (does any template ???)

[jbemmel]

Just some minor structural changes suggested:

{% if (STP disabled) %}
no spanning-tree vlan 1-4094
{% else %}
# Rest of the template
{% endif %}

.. although I suppose in theory one could disable stp at the top level, and enable it for a particular vlan or interface.

Up to you if you think that should be supported

[DanPartelly]

We have in docs STP Docs in section Global, Node, Link, Interface, and VLAN Parameters a tip: You can set the ‌stp.enable parameter in the ‌vlans dictionary to enable per-VLAN STP.

So ... Anyways, Ill change it to your preferred version since you wrote the STP module and you know best what where your intentions behind some choices.

[DanPartelly]

Default pvrstp / pvstp to long pathcost method,

[ipspace]

Looks good to me (but then mercifully I know nothing about STP), and the integration tests pass.