Introduce netplan validate

[daniloegea]

Description

netplan validate will parse and report any issues in the current configuration files without generating or applying it to the system.

The --debug parameter can be used to show what files are being loaded and in what order. It will also show any shadowing that happened during the parsing. A useful feature during troubleshooting. It implements the same logic from:

netplan/src/util.c

Line 665 in 4e89bbf

/* Files with asciibetically higher names override/append settings from

Example:

# netplan validate --debug --root-dir /tmp/dir
Order in which your files are parsed:
/tmp/dir/etc/netplan/00-ethernet-set-name.yaml
/tmp/dir/run/netplan/00-network.yaml
/tmp/dir/etc/netplan/01-ethernet-set-name-and-match.yaml
/tmp/dir/etc/netplan/02-bridge.yaml
/tmp/dir/run/netplan/03-bridge.yaml
/tmp/dir/etc/netplan/03-ethernet.yaml
/tmp/dir/run/netplan/bad_boolean.yaml
/tmp/dir/run/netplan/bad_ethernet.yaml
/tmp/dir/run/netplan/bad_mac_address.yaml
/tmp/dir/run/netplan/bad_vlan.yaml
/tmp/dir/etc/netplan/change_type.yaml
/tmp/dir/etc/netplan/config.yaml
/tmp/dir/run/netplan/duplicate_addresses.yaml
/tmp/dir/run/netplan/good_ethernet.yaml
/tmp/dir/run/netplan/good_vlan.yaml
/tmp/dir/run/netplan/invalid_value.yaml
/tmp/dir/run/netplan/out_of_range.yaml
/tmp/dir/etc/netplan/wireless.yaml

The following files were shadowed:
/tmp/dir/lib/netplan/03-ethernet.yaml shadowed by /tmp/dir/etc/netplan/03-ethernet.yaml
/tmp/dir/etc/netplan/03-bridge.yaml shadowed by /tmp/dir/run/netplan/03-bridge.yaml
/tmp/dir/etc/netplan/00-network.yaml shadowed by /tmp/dir/run/netplan/00-network.yaml

Validation failed: /tmp/dir/etc/netplan/00-ethernet-set-name.yaml:4:7: Error in network definition: enp3s0: 'set-name:' requires 'match:' properties
      set-name: "external"
      ^

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[blackboxsw]

Is it worth growing generally a machine readable output so callers can invoke this with a --format=json or do we really prefer machine-readable consumers really to just use the libnetplan.State.import_parser_results directly and process the state errors?