use default resources for testing

[Samuel Allan]

A common pattern of k8s charms is to specify container images in metadata.yaml, with a hint at the upstream source of the images like so:

```
resources:
  mysql-image:
    type: oci-image
    description: Ubuntu LTS Docker image for MySQL
    upstream-source: dataplatformoci/mysql-and-shell:latest
```

This is all fine for deploying the charm from charmhub, because charmhub should be hosting the image as resource and juju can find it. But for deploying a local charm, juju requires providing sources for all the resources on the command line:

```
juju deploy ./mysql-k8s_ubuntu-20.04-amd64.charm --resource mysql-image=dataplatformoci/mysql-and-shell:latest
```

Same goes for local charms in bundle files:

```
  mysql:
    charm: ./mysql-k8s.charm
    resources:
      mysql-image: dataplatformoci/mysql-and-shell:latest

```

For local development and testing, this is verbose (especially for charms with multiple containers) and redundant, because most of the time we're copy/pasting from the upstream-source value in metadata.yaml.

It would be nice to have some short flag to direct juju to use these default / upstream resources, so a deployment could look something like this:

```
juju deploy ./mysql-k8s.charm --default-resources
```

or

```
mysql:
  charm: ./mysql-k8s.charm
  default_resources: true
```

Note that it's not clear if `upstream-source` key in metadata.yaml resources is officially part of the spec, but it appears to be at least a defacto standard across recent k8s charms.