Add support to use current node parameters as references in class name

so that with the following reclass config:

```
=> /etc/reclass/nodes/mynode.yml
classes:
  - common
parameters:
  project: myproject

=> /etc/reclass/classes/common.yml
class:
  - ${project}

=> /etc/reclass/classes/myproject.yml
parameters:
  some:
    project: parameters
```

The mynode parameters would be merged successfully by reclass and we will
get the following result for the parameters:

```
parameters:
  project: myproject
  some:
    project: parameters
```

Without this patch, this would result in the following error:

```
Failed to load ext_pillar reclass: ext_pillar.reclass: -> mynode
	In yaml_fs:///etc/reclass/classes/common.yml
	Class name ${project} not resolvable
```
1 file changed