DVPL-5428 Add error message when SPLUNK_HOME not defined

We now produce a warning message with a recommendation at the start of SearchCommand.process:

{code}
        if u'SPLUNK_HOME' not in environ:
            self.logger.warning(
                u'SPLUNK_HOME environment variable is undefined.\n'
                u'If you are testing outside of Splunk, consider running under control of the Splunk CLI:\n'
                u'    splunk cmd %s\n'
                u'If you are running inside of Splunk, SPLUNK_HOME should be defined. Consider troubleshooting your '
                u'installation.', self)
{code}