Store plain free text to GERRIT_CHANGE_COMMIT_MESSAGE

GERRIT_CHANGE_COMMIT_MESSAGE is now Base64 encoded to prevent escape issues.
But this parameter value is shown in tooltip queue list has. So it might be better to store plain text into this parameter. 

This patch does it then replace the class for this multiline parameter from StringParameterValue to TextParameterValue. This affects parameter view only.

Unfortunately, the below issues are remained.

1. All text are shown in tooltip
   * This is TextParameterValue bug in core. getShortDescription() returns multiline text, but spec of this method is singleline.
2. Escape issue.
   * I whould like to introduce my plugin[1] I believe this can solve escape related issue.
3. Change Base64 string to plain text
   * Umm.. backward compatibility is broken... Can you accept?

[1] https://wiki.jenkins-ci.org/display/JENKINS/Export+Parameters+Plugin