Make source fields required for 'Create Volume' form

Raise an error message when 'Volume Source' field has been
changed to 'Image', 'Volume' or 'Snapshot' but no actual
image/volume/snapshot has been selected.

Closes-Bug: #1348760

Change-Id: I2d864c78ba2f5bce55f6fe77075ea6dd848f0f6a
Merge "Make source fields required for 'Create Volume' form"