Cloud provider should keep the AZ for ILB frontendIPConfiguration

[feiskyer]

What happened:

After #403, cloud provider would always set AZ when a region supports AZ. But because a region may switch from non-AZ to AZ, old frontendIPConfiguration without AZ would get ResourceAvailabilityZonesCannotBeModified error when the cloud provider azure is upgraded to v0.7+ on the AZ supported regions.

What you expected to happen:

The old AZ values on ILB frontendIPConfiguration should be kept when upgrading cloud provider Azure versions.

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• OS (e.g: cat /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Network plugin and version (if this is a network-related bug):
• Others:

[feiskyer]

/assign @nilo19