Fixing Inject to not create empty tags

Using the Inject functionality with the builders section caused
an error in Jenkins because the script-file was blank but the tag
existed in the xml.

Fixing this by adding a function to create the XML only if the given
value is not None and not a blank value '':

    def add_nonblank_xml_subelement(parent, tag, value):

Change-Id: I9151f7444f6807e929721b04fb8e7ad3a867cd6c
Reviewed-on: https://review.openstack.org/18820
Reviewed-by: Jon-Paul Sullivan <jonpaul.sullivan@hp.com>
Reviewed-by: Tollef Fog Heen <tfheen@varnish-software.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins