Since the trusted_image_certificates was added to nova[1], not only
nova-compute but also nova-api uses this parameter.
This change migrates the puppet parameter from nova::compute class
to more common nova::glance class, so that the parameter can be set
at nova-api as well.
[1] 8c7ca368b190f0fd3c097531e2cf52fe4dc20c69
Change-Id: I8c7cf5269cda8b6a8e0a22dbb326454d923fb412