Fixed "File Not Found" error on image in config-reference.

Configuration Reference build were displaying an error because one of
the references to an image in common content was missing an additional
level of the relative path:

[error] File not found:
'/home/sgordon/Source/openstack-manuals/doc/config-reference/common/figures/filteringWorkflow1.png'.
File is referred in
'/home/sgordon/Source/openstack-manuals/doc/config-reference/compute/section_compute-scheduler.xml'
fileRef='../common/figures/filteringWorkflow1.png'.

The fix was to change ../common/figures/ to ../../common/figures,
accounting for the additional level added to the path by the "compute"
directory in the configuration reference guide.

Change-Id: Ie3a4d8debc1008377ed90ba01bbf3a50fdef6a85
Merge "Fixed "File Not Found" error on image in config-reference."