devstack-plugin-ceph: Remove DEVSTACK_GATE_CEPH for ceph plugin job
devstack-plugin-ceph: Remove DEVSTACK_GATE_CEPH for ceph plugin job

For the ceph plugin job, all the settings are sourced from the
plugin's repo files, thus we no longer need to set
DEVSTACK_GATE_CEPH in the plugin's job-template definition

Change-Id: I857e5091df7f3b0ca610ca437bddf596d83ceb9d

openstack-infra/project-config,dongwenjuan/project-config,Tesora/tesora-project-config,noorul/os-project-config,dongwenjuan/project-config,noorul/os-project-config,openstack-infra/project-config,Tesora/tesora-project-config