run script and load properties in correct order

By default the EnvInject plugin loads the properties
file before running the bash script when defined in wrapper
section.  However we want the opposite execution order, run script
then load properties file.

To work around this ordering problem we need to define
two instances of EnvInject, 1st instance to run the
script and 2nd instance to load the properties file.
This cannot be done in jenkins UI, but apparently works
when defined in JJB.

This will also let us run gerrit-git-prep in the
more conventional pre-builder step.

Change-Id: I7a16143a2f9b79d2dbe2684a7d016f6640f5a707
Reviewed-on: https://review.openstack.org/34936
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: matthew wagoner <zxkuqyb@gmail.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins