Added contentwidth attibute to images for pdf builds

contentwidth="6in" was added (or modified from scale="60") to imagedata elements
in the following files:
  doc/src/docbkx/common/dashboard_customizing.xml
  doc/src/docbkx/common/getstart.xml
  doc/src/docbkx/common/trusted-compute-pools.xml
  doc/src/docbkx/common/vmware.xml
  doc/src/docbkx/openstack-compute-admin/computeconfigure.xml
  doc/src/docbkx/openstack-compute-admin/computevolumes.xml
This makes big images fit in correctly in pdf builds.

Change-Id: I0d3a410d043be93a94d04842603428368e4d78ca
Merge "Added contentwidth attibute to images for pdf builds"