tests: allow setting KUDU_ALLOW_SLOW_TESTS to false values
tests: allow setting KUDU_ALLOW_SLOW_TESTS to false values

Previous to this commit, if this environment variable was set at all,
it would be interpreted as true, even if set to values like "false",
"0", or "no".

Change-Id: Id5932a7e8b0fe696fa7d2a8a2aff87df44f539c6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/409
Tested-by: jenkins
Reviewed-by: Eli Collins <3ca62a2ad87376be2697209a1770e1afdbbba220@cloudera.com>

EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu