Add support for multiple custom_structured_configuration prefixes

[ClausHolbechArista]

Change Summary

Allow multi layered custom_structured_configuration and more flexible naming.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Documentation content changes
• Other (please describe):

Related Issue(s)

Component(s) name

eos_l3ls_evpn

Proposed changes

custom_structured_configuration_prefix allows the user to customize the prefix for Custom Structured Configuration variables.
Default value is custom_structured_configuration_. Default value is custom_structured_configuration_. Remember to include any delimiter like the last _ in this case.
It is possible to specify a list of prefixes, which will all be merged one by one. The order of merge will start from beginning of the list, which means that keys defined in the later prefixes will be able to override keys defined in previous ones.

custom_structured_configuration_prefix: < variable_prefix, default -> "custom_structured_configuration_" >	custom_structured_configuration_prefix: < variable_prefix, default -> "custom_structured_configuration_" >
#or
custom_structured_configuration_prefix: [ < variable_prefix_1 > , < variable_prefix_2 > , < variable_prefix_3 > ]

Example using multiple prefixes:

custom_structured_configuration_prefix: [ my_dci_ , my_special_dci_ ]

my_special_dci_ethernet_interfaces:
  Ethernet4000:
    ip_address: 10.3.2.1/21

my_dci_ethernet_interfaces:
  Ethernet4000:
    description: My test
    ip_address: 10.1.2.3/12
    shutdown: false
    type: routed
    mtu: 1500
    peer: MY-own-peer
    peer_interface: Ethernet123
    peer_type: my_precious

In this example Ethernet4000 will be added to the ethernet_interfaces dictionary in the Structured Configuration and the ip_address will be 10.3.2.1/21 since ip_adddress was overridden on the later custom_scructured_configuration_prefix

How to test

Tested with my test repo: https://github.com/ClausHolbechArista/claus-l3ls-dev/commits/superspine-alternative
Following commits show each test scenario.

• Test without custom config
• Test with default prefix and custom config
• Test with 1 custom prefix but no matching config
• Test with 1 custom prefix and both matching and non-matching custom config (non-matching is just ignored)
• Test with multiple custom prefixes. One using the default name and others using custom names. Also including overrides.

Checklist:

• My code has been rebased from devel before I start
• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have updated molecule CI testing accordingly
• All new and existing tests passed (pre-commit, make linting and make sanity-lint).

[titom73]

Looks good to me

[ClausHolbechArista]

Rebased and refreshed CI artifacts