[installer] Add flag to configure strict config parsing

[easyCZ]

Description

To make it possible to run config validation in non-strict mode. Internal context.

Related Issue(s)

Fixes #

How to test

$ cd installer/install
$ go run main.go init > config.yaml

# set domain
$ yq w -i config.yaml '.domain = "foo.bar"' 

$ echo "foo: bar" >> config.yaml

$ go run main.go validate config --config ./config.yaml
# expected failure, the current and default behaviour
Error: error loading config: error unmarshaling JSON: while decoding JSON: json: unknown field "foo"

$  go run main.go validate config --config ./config.yaml --strict-parse=false 
{
  "valid": true
}

Release Notes

[installer] Command line flag to configure strict config parsing

Documentation

NONE

[geropl]

nit: being more explicit of what "strict" or "non-strict" means would be nice. E.g. I had to look up the docs to learn that strict fails on a) unknown and b) double fields.

[geropl]

Tested and works. ✔️