IMPALA-7199: Add scripts to create code coverage reports

gcovr is a python library that uses gcov to generate
code coverage reports. This adds gcovr to the python
dependencies and adds bin/impala-gcovr to provide
easy access to gcovr's command line. gcovr 3.4
supports python 2.6+.

This also adds bin/coverage_helper.sh to provide a
simplified interface to generate reports and zero
coverage counters.

Code coverage data is written out when a program
exits, so it is important to avoid hard kills
to shut down the impalads when generating coverage.
This modifies testdata/bin/kill-all.sh to call
start-impala-cluster.py --kill when shutting down
the minicluster to try to avoid doing a hard kill.
It will still do a hard kill if impala is still
running after the softer kill.

Change-Id: I5b2e0b794c64f9343ec976de7a3f235e54d2badd
Reviewed-on: http://gerrit.cloudera.org:8080/10791
Reviewed-by: Joe McDonnell <joemcdonnell@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>

infra/python/deps/requirements.txt

@@ -43,6 +43,7 @@ Flask == 0.10.1
   MarkupSafe == 0.23
   Werkzeug == 0.11.3
   itsdangerous == 0.24
+gcovr == 3.4
 kazoo == 2.2.1
 ordereddict == 1.1
 pexpect == 3.3