Workaround to replace wrong lease file path given by default
in kea-1.6.0-4.el7.x86_64.rpm built for CentOS

See https://gitlab.isc.org/isc-projects/kea/issues/1144

ref: https://gitlab.isc.org/isc-projects/kea/issues/208

ref: https://oldkea.isc.org/ticket/5338

ref: https://github.com/moby/moby/issues/20037

As recommended by official documentation

- use e instead of error
- add FrozenStringLiteralComment
- fix hash alignment
- prefer string interpolation to string concatenation
- prefer transform_values over map {...}.to_h
- convert if-elsif to case-when

Add systemd timer unit that calls a shell script to generate dhcp client
metrics in prometheus format intended to be collect by prometheus node
exporter. Only IP lifetime of dynamically assigned network interface is
currently reported.

Use the first key of a value to override it in attribute array. To test
it, all test configuration are set in a role (node can only have normal
attributes).

For instance:
"logguers": [
  { name: first, ... }
]
If we define another logguers with name: first, then the second one will
override the first one, which is defined in attribute default.