Add support fo specifying the manifest file in the CR

At the moment, if we need to change the mdc deployment config to mount the service manifest file, we have do it manually, see https://github.com/integr8ly/installation/blob/master/roles/mdc/library/mdc_patch_manifest.py.

Ideally, this should be handled by the operator itself: the manifest file name can be specified either in the CR or through a env var, and the operator will modify the mdc deployment config to use it.

ping grdryn lfitzger@redhat.com