Implement ValidateVolumeCapabilities and refactor parameter handling for more comprehensive validation of existing disks in all cloud calls

[davidz627]

TODO:

• Add some unit tests for new functions :)

/kind cleanup
/kind feature
/assign @msau42 @misterikkit

Partially does #384 (Still need to do Node side validation in a seperate PR)
Fixes #162

ValidateVolumeCapabilities validates that the given volume conforms to all capabilities in the request. Validation of existing volumes during inserts also improved to check all parameters.

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[msau42]

How much of this is going to conflict with #415? Should we try getting that one in first?

[msau42]

should it be considered an error if a user specified a key without a value?

[davidz627]

in-tree PD just treats that case as the default. I tried setting type: "" and it just created a pd-standard without complaining - I think we should be backwards compatible here

[davidz627]

#415 seems to have stalled and already has conflicts

[davidz627]

@msau42 added unit tests, PTAL

[msau42]

/lgtm