add psmisc package to build slaves
add psmisc package to build slaves

Provides `killall` need by the stack-os-matrix post-build cleanup step.  psmisc
is part of @base but is not present in the bento el7 builds [reason unknown].

jhoblitt/sandbox-jenkins-demo,jhoblitt/sandbox-jenkins-demo,lsst-sqre/sandbox-jenkins-demo,lsst-sqre/sandbox-jenkins-demo