Fix session management in manual trigger page

In ManualTriggerAction, session is always created if nothing.
It might be issue if session is closed before build.

This patch creates session once then reuse it.

Fix for JENKINS-19013

Task-Url: https://issues.jenkins-ci.org/browse/JENKINS-19013
Merge pull request #176 from rinrinne/session-manage

Fix session management in manual trigger page
Provide notification level to gerrit command

Now notification level feature is provided to REST API only.

This patch provides this feature to Gerrit command as parameter
named 'NOTIFICATION_LEVEL'.

Note that this parameter always 'ALL' at build started.
It's a default value if ReviewInput has no Notify attribute.
Merge pull request #177 from rinrinne/notify-for-ssh

Provide notification level to gerrit command