Call "lftools jenkins" after credentials are set

Call "lftools jenkins" after credentials are set

We have been seeing failures due to the "lftools jenkins plugins"
call being made without credentials being set first. The previous
method did not require credentials, so the failure was introduced
when we switched to using lftools. The os_plugin_version variable
is not needed before this part of the script, so by moving the
assignment to after JENKINS_USER and JENKINS_PASSWORD are set, the
script is once again successful.

Issue: IT-18593
Change-Id: Ib3e6099992abade5860eedd1bda07015b13f0f91
Signed-off-by: Eric Ball <eball@linuxfoundation.org>