Document and make OS_CACHE work

Before the --os-no-cache option logic would overwrite any value set in env['OS_CACHE'].
Since os-no-cache  is deprecated, and also the default value, just remove it.
Also update the help message for --os-cache to mention the environment variable.

Change-Id: Ie2569d5cdf0f12f3a29e9a6f20faef75956d2209
Merge "Document and make OS_CACHE work"