Add optional Hub label for tunnel naming

[dmitryperets]

The default naming convention for the tunnels generated on the Spokes is:

H<hub_index>_<ol_type>

For example, for a Dual-Hub topology we will generate tunnels like H1_INET and H2_INET.

To simplify operations, we introduce an option to set a more meaningful label for each Hub, using an optional hub_name parameter in the hubs structure:

{# Hubs #}
{% set hubs = {

    'site1-H1': {
      'lo_bgp': '10.200.1.253',
      'hub_name': 'TRN',          <<<<<<
      'overlays': {
         ...

If this option is defined, the naming convention will be:

<hub_name>_<ol_type>

In other words: TRN_INET.

Similarly, this optional name will be used for the Hub-to-Hub tunnels in a multi-regional deployment (e.g. EAST_TRN_INET).