DRILL-8175: Update Drill release script after 1.20 (#2503)
* Set DRILL_PID_DIR in Dockerfile to writable location for distributed mode.
Some users of the images built from this Dockerfile customise
them so that they launch Drill in distributed mode instead of
embedded mode. This change saves them from having to set
DRILL_PID_DIR themselves in order to succeed.
* Update release script and instructions after the release of 1.20.
- Add support for specifying a build profile such as "hadoop-2".
- Update instuctions for the Drill web site.
- Update instructions for uploading RCs (no more home.apache)
- Some fixes.
