Allow changing the collectors that are used.

Positional arguments now specify which collectors to use. This allows
disabling a collector if it is problematic, and also re-ordering of the
emitted $OS_CONFIG_FILES from the default order if necessary.

Change-Id: I7e76db991c0b16c529c1cbf9a1ba9beb78e45482
Merge "Allow changing the collectors that are used."