reduce dfs.namenode.safemode.extension

Reduce dfs.namenode.safemode.extension so that we don't linger in
safemode for a long time during testing.

Signed-off-by: Colin McCabe <cmccabe@alumni.cmu.edu>
core-site.xml: add fs.tachyon.impl

Needed to handle tachyon:// urls.

Signed-off-by: Colin McCabe <cmccabe@alumni.cmu.edu>
spark-env.sh: adjust memory, add tachyon stuff

Signed-off-by: Colin McCabe <cmccabe@alumni.cmu.edu>
yarn-site.xml: some debugging settings

* Set yarn.log-aggregation-enable = false to keep logs separate
(although this is the default currently).

Set yarn.scheduler.minimum-allocation-mb and
yarn.scheduler.maximum-allocation-mb, to set up a sensible range of
memory each task should use.

Set yarn.resourcemanager.am.max-attempts to 1 to avoid seeing so many
retry attempts.

Signed-off-by: Colin McCabe <cmccabe@alumni.cmu.edu>