Prometheus dashboards are tied to "Local" datasource

The ansible installer configures a "prometheus" datasource, so lots of dashboards fail to load due to the missing data. These should either just use the "default" datasource or be switched to "prometheus".

```
[mgr-prometheus]$ git grep Local
alert-status.json: "name": "Local",
ceph-at-a-glance.json: "name": "Local",
ceph-backend-storage.json: "name": "Local",
ceph-cluster.json: "name": "Local",
ceph-cluster.json: "datasource": "Local",
ceph-health.json: "name": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-osd-information.json: "datasource": "Local",
ceph-pools.json: "name": "Local",
ceph-rgw-workload.json: "name": "Local",
iops-by-server.json: "name": "Local",
iscsi-client-details.json: "name": "Local",
iscsi-overview.json: "name": "Local",
iscsi-overview.json: "datasource": "Local",
iscsi-overview.json: "datasource": "Local",
iscsi-overview.json: "datasource": "Local",
iscsi-overview.json: "datasource": "Local",
iscsi-overview.json: "datasource": "Local",
latency-by-server.json: "name": "Local",
mds-performance.json: "name": "Local",
network-usage-by-node.json: "name": "Local",
network-usage-by-node.json: "datasource": "Local",
network-usage-by-node.json: "datasource": "Local",
osd-node-detail.json: "name": "Local",

```