add extend volume tests:

- test_volume_extend
- test_volume_extend_with_size_smaller_than_original_size
- test_volume_extend_with_non_number_size
- test_volume_extend_with_None_size
- test_volume_extend_with_nonexistent_volume_id
- test_volume_extend_without_passing_volume_id

Closes-Bug: #1214251

Change-Id: I05d96109fdbdae47949126e852db6f274e3c2c4f
Merge "add extend volume tests:"