Remove defaults for architectures and tags

Hi,

To make svammel generic the defaults should be removed and all necessary information should have to be specified explicitly. The reason for this is that svammel is used as a fail reporter for i386 and not only for arm porting issues.

The new use case gets a bit confusing as I have left the defaults to the original required tags and archs. If this is fixed the following options will change behavior:
    --bugtag (one or more to be specified, can be omitted to have no tags set)
    --target-arch (a value must be supplied)
    --reference-arch (one or more to be specified, can be omitted to have all bugs reported, regardless of possible successful builds on other architectures)

If no bugtags are specified, checking for already filed bugs gets tricky, so perhaps at least one tag should be required?

Thanks,

Mattias