stackset deploy: option for strict config parsing

[dhx-mike-palandra]

$ rain --version
Rain v1.24.2 linux/amd64

If a file passed to --config is valid YAML, then unrecognized keys are silently ignored in some cases.

Example:

I was trying to enable "managed execution" on a StackSet. According to the sample config linked from rain stackset deploy --help, the (incomplete) syntax is:

StackSet:
    managedexecution: null   

The actual syntax is one of:

StackSet:
    managedexecution:
        active: false|true

While figuring that out, I tried Active (instead of active). That syntax error was silently ignored even with --debug.

Related to: #748, #749.