isis: T5335: fix invalid isis config base in migration script

[jvoss]

Change Summary

This PR resolves an issue when loading or merging configuration files containing IS-IS configuration items. Before this change an error in the migration script (/opt/vyatta/etc/config-migrate/migrate/isis/0-to-1) is experienced:

/tmp/test_isis.conf

interfaces {
  loopback lo {
  }
}
protocols {
  isis {
    interface lo {
    }
    net 49.0001.0000.0000.0001.00
  }
}

jvoss@vyos# merge /tmp/test_isis.conf

Migration script error: /opt/vyatta/etc/config-migrate/migrate/isis/0-to-1: [Errno 1] failed to run command: ['/opt/vyatta/etc/config-migrate/migrate/isis/0-to-1', '/tmp/tmpd0jll6fb']
returned: 
exit code: 1.
[edit]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T5335

Component(s) name

• IS-IS

Proposed changes

Update the configuration base used in the migration script to start at protocols isis instead of considering additional nodes that may be present in the proposed configuration file.

How to test

Create configuration file /tmp/test_isis.conf:

interfaces {
  loopback lo {
  }
}
protocols {
  isis {
    interface lo {
    }
    net 49.0001.0000.0000.0001.00
  }
}

Attempt to load the configuration:

jvoss@vyos# merge /tmp/test_isis.conf

Migration script error: /opt/vyatta/etc/config-migrate/migrate/isis/0-to-1: [Errno 1] failed to run command: ['/opt/vyatta/etc/config-migrate/migrate/isis/0-to-1', '/tmp/tmpd0jll6fb']
returned: 
exit code: 1.
[edit]

Make the proposed changes in this PR, then attempt to load and again:

jvoss@vyos# merge /tmp/test_isis.conf
Merge complete. Use 'commit' to make changes effective.
[edit]

jvoss@vyos# compare
[protocols]
+ isis {
+     interface     lo { }
+     net "49.0001.0000.0000.0001.00"
+ }

[edit]

Commit changes:

jvoss@vyos# commit
[edit]

Validate IS-IS is now running:

jvoss@vyos:~$ show isis interface 
Area VyOS:
  Interface   CircId   State    Type     Level
  lo          0x0      Up       loopback L1L2 

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly