fix(module/vmseries): fix destroy sequence for LB backends

[FoSix]

Description

For destroy operations, force the LB backend association to be destroyed 1^st.

Motivation and Context

W/o that manual dependency, during destroy operations, a race condition can happen, when VM is being destroyed before the NIC - LB backend association. In such case the destroy operation fails with an error. The problem is not the order but the fact that the VM is in a 'destroying' phase while the the destroy operation is run on the association.

This fix can also close #199 as the problem described in that issue is triggered only when a destroy operation 1^st fails due to the described race condition.

How Has This Been Tested?

Existing code/example have been run to prove that the race condition cannot be reproduced.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.