Merge branch '14451-crunchstat-summary-cache-ram'

Fix three small crunchstat-summary issues:
- Use default of 1 if no cores requested. fixes #12026
- Don't recommend RAM or CPU changes based on zero usage. fixes #10570
- Remove extra MB (2**20) factor in Keep cache recommendation.  Fixes #14451

Arvados-DCO-1.1-Signed-off-by: Tom Morris <tfmorris@veritasgenetics.com>