Move plugin config to the relevant plugin instead of @ top-level

[ekinanp]

Consider the following wash.yaml file

external-plugins:
  - script: 'foo'
plugins:
  - 'bar'
foo:
  <foo_config>
bar:
  <bar_config>

Having the plugin config @ top-level is a bit confusing. We should change this to something like

external-plugins:
  - script: 'foo'
    config: <config stuff>
plugins:
  - name: 'foo'
    config: <config stuff>

Note that this would also let us configure an external plugin's name so that it doesn't have to be <script_without_extension>. E.g., we could do something like

external-plugins:
  - script: 'bar'
    name: 'foo'
    config: <config>

And Wash would know that the plugin's name is foo.