Add optional support to ignore classes that are not found

[bbinet]

I'd like to configure reclass so that it ignores possible missing classes.

[bbinet]

Do not merge: this is not ready yet.

[bbinet]

Actually this is now ready to be reviewed.

[bbinet]

Bump :)

[fpytloun]

I like the idea and can imaging the use-case - eg. include class that gets generated on demand (eg. by deploy tool) and can override some parameters. But it may be good to provide some logging (eg. "Inclusion of slass xyz skipped because it doesn't exist") to make debugging with this option enabled easier :-)

[bbinet]

Sure, what is the normal way to log messages in reclass?