VolumeMount should require Volume component definition

[kadel]

What does this PR do?

VolumeMount should require Volume component definition.
Otherwise, it doesn't make much sense to add it implicitly.
One of the reasons is that could lead to errors like this:

components:
  - container:
      name: cont1
      volumeMounts:
        - name: myvolume
  - container:
      name: cont2
      volumeMounts:
        # I want to use myvolume, but made a typo
        - name: myvoluem

volumes:
   - name: myvolume

What issues does this PR fix or reference?

Is your PR tested? Consider putting some instruction how to test your changes

Docs PR

[maysunfaisal]

@l0rd @davidfestal when and how is the schema here updated? https://devfile.github.io/devfile/_attachments/api-reference.html