ServiceAccountName not properly set in juju 2.8

[David]

Hello,

A K8s charm with RBAC is not working in juju 2.8.

How to reproduce:

```
$ sudo snap install juju --classic --channel 2.8/stable
$ sudo snap install microk8s --classic
$ microk8s.status --wait-ready
$ microk8s.enable rbac storage dns

$ juju bootstrap microk8s
$ juju add-model osm
$ juju deploy cs:~charmed-osm/mongodb-k8s --config enable-sidecar=true

$ watch -c juju status --color # Wait until it is ready
$ microk8s.kubectl -n osm logs mongodb-k8s-0 -c mongodb-sidecar-k8s
Error in workloop { [Error: [object Object]]
  message:
   { kind: 'Status',
     apiVersion: 'v1',
     metadata: {},
     status: 'Failure',
     message:
      'pods is forbidden: User "system:serviceaccount:osm:default" cannot list resource "pods" in API group "" in the namespace "osm"',
     reason: 'Forbidden',
     details: { kind: 'pods' },
     code: 403 },
  statusCode: 403 }
$ microk8s.kubectl -n osm get pods mongodb-k8s-0 -o json | jq .spec.serviceAccountName
"default"
```

Doing the same with juju 2.7, it's working perfectly:

```
$ sudo snap install juju --classic --channel 2.7/stable
$ sudo snap install microk8s --classic
$ microk8s.status --wait-ready
$ microk8s.enable rbac storage dns

$ juju bootstrap microk8s
$ juju add-model osm
$ juju deploy cs:~charmed-osm/mongodb-k8s --config enable-sidecar=true

$ watch -c juju status --color # Wait until it is ready
$ microk8s.kubectl -n osm get pods mongodb-k8s-0 -o json | jq .spec.serviceAccountName
"mongodb-k8s"
```

I noticed that the serviceAccountName is "default" in 2.8, but it is "mongodb-k8s" in 2.7.

[Pen Gale (pengale)]

This is currently blocking the OSM team on a deployment.

[Yang Kelvin Liu (kelvin.liu)]

https://github.com/juju/juju/pull/12141 will be landed to 2.8 to fix this issue