piuparts_wrapper: unset scripts dir when non existent
piuparts_wrapper: unset scripts dir when non existent

On Ubuntu Precise (Long Term Support release), the piuparts script does
not comes with /etc/piuparts/scripts which causes the piuparts output to
report:

 0m0.0s ERROR: Scripts directory is not a directory:
 /etc/piuparts/scripts/

Add a check to make sure it is a directory, if it is not, fallback to an
empty string passed to piuparts.

hashar/jenkins-debian-glue,mika/jenkins-debian-glue,vorlock/jenkins-debian-glue,n054/jenkins-debian-glue,vorlock/jenkins-debian-glue,zeha/jenkins-debian-glue,n054/jenkins-debian-glue,zeha/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,jbfavre/jenkins-debian-glue,tonin/jenkins-debian-glue,jbfavre/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,hashar/jenkins-debian-glue,cclausen/jenkins-debian-glue,mika/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,kartikm/jenkins-debian-glue,tonin/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,cclausen/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,kartikm/jenkins-debian-glue