[BEAM-1621] Add streaming executions to examples/java integration tests

[kennknowles]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• Make sure tests pass via mvn clean verify. (Even better, enable
  Travis-CI on your fork and ensure the whole test matrix passes).
• Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
• If this contribution is large, please file an Apache
  Individual Contributor License Agreement.

---

[coveralls]

Coverage remained the same at 69.894% when pulling 00a0ff8a4c8058f06209811bb31fc9594e90916e on kennknowles:streaming-example-ITs into 1783819 on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8090/

Failed Tests: 1

beam_PreCommit_Java_MavenInstall/org.apache.beam:beam-examples-java: 1

• org.apache.beam.examples.WordCountIT.testE2EWordCount

--none--

[kennknowles]

R: @davorbonaci @jasonkuster this isn't great if the JVM gets killed, but on balance I am for adding this minimal coverage now and improving the safety later

CC: @tgroh looks like this would be enough to catch the streaming side input issue. Rebased on top of your fix.

[coveralls]

Coverage increased (+0.003%) to 70.101% when pulling b58117c on kennknowles:streaming-example-ITs into 3fde7b2 on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8099/
--none--

[davorbonaci]

LGTM.