Improve documentation for integration tests

The integration tests section in http://samza.apache.org/contribute/tests.html should contain the following information:

• SSH server should be running on the local machine if running the integration tests locally
• On Linux, the user may edit the ~/.bashrc file (or other shell specific configuration file) to configure JAVA_HOME. Zopkio is doing remote deployment via SSH in non-interactive mode, therefore the configuration from /etc/profile and other configuration files aren't taken into account.