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