add el7 fixups

* cinder/manifests/backend/iscsi.pp cinder/manifests/volume/iscsi.pp
  switch to LIO for el7
  To be resolved upstream: why cinder puppet module has iscsi in two places??

* mysql/manifests/params.pp
  force mariadb for el7

* mysql/templates/my.cnf.erb
  my.cnf template was broken for el7 mariadb
  pid-file was missing in [mysqld_safe] causing startup failure:
  Process: 5140 ExecStartPost=/usr/libexec/mariadb-wait-ready $MAINPID (code=exited, status=1/FAILURE)
  Process: 5139 ExecStart=/usr/bin/mysqld_safe --basedir=/usr (code=exited, status=0/SUCCESS)
  Process: 5117 ExecStartPre=/usr/libexec/mariadb-prepare-db-dir %n (code=exited, status=0/SUCCESS)
  Main PID: 5139 (code=exited, status=0/SUCCESS)

* nova/manifests/compute/libvirt.pp
  remove messagebus->libvirt check for el7
  also wrong assumption for el7, that ! Fedora assumes RHEL _6_
  was failing with
  Error: /Stage[main]/Nova::Compute::Libvirt/Service[libvirt]: Could not evaluate: Could not find init script for 'libvirtd'