Schema error messaging doesn't handle 'anyof' with 'required'

An example under the `cc_yum_add_repo` properties:
```
"anyOf": [
{
"required": [
"baseurl"
]
},
{
"required": [
"metalink"
]
}
]
```

This provides the desired validation result, but if you exclude both baseurl and metalink, the error doesn't include both properties:
```
# cloud-init schema -t cloud-config -c a.yaml
Invalid user-data a.yaml
Error: Cloud config schema errors: yum_repos.epel-testing: 'baseurl' is a required property

Error: Invalid schema: user-data
```

We should have a way of messaging that `baseurl` or `metalink` is required, not just `baseurl`.

Reproduce the reported case with `cloud-init schema -t cloud-config -c a.yaml` using the `cc_yum_add_repo` properties example. Trace the schema validation error handling for `anyOf` entries containing `required`; done means the error clearly reports that either `baseurl` or `metalink` is required.

Written by the indexing model from the issue text.