cleanup keywords used/available in hotspot testbase

test/hotspot/jtreg/TEST.ROOT specified the following k/w as available:
cte_test jcmd nmt regression gc stress metaspace headful intermittent randomness

some of them don't seem to be very useful, e.g. gc which is used only by obvious gc tests, some are barely used, e.g. metaspace which has only 2 tests. this RFE is to clean this up which includes but not limited to removal of redundant k/w, documenting others.