fix controller manager crash issue on a manually created azure k8s cluster

[andyzhangx]

What this PR does / why we need it:
fix controller manager crash issue on a manually created k8s cluster, it's due to availability set nil issue in azure loadbalancer

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #
In the testing of a manually created k8s cluster, I found controller manager on master would crash in current scenario:

1. Use acs-engine to set up k8s 1.7.7 cluster (it's with an availability set)
2. Manually add a node to the k8s cluster (without an availibity set in this VM)
3. Set up a service and schedule the pod onto this newly added node
4. controller manager would crash on master because although this k8s cluster has an availability set, the newly added node's machine.AvailabilitySet is nil which would cause controller manager crash

Special notes for your reviewer:
@brendanburns @karataliu @JiangtianLi

Release note:

Azure cloudprovider: Fix controller manager crash issue on a manually created k8s cluster.

/sig azure

[jdumars]

/lgtm

[andyzhangx]

Hi @jingxu97 , could you take a look, thx

[andyzhangx]

@jdumars could you approve by /approve no-issue, thx

[jdumars]

/approve no-issue

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyzhangx, jdumars

Associated issue requirement bypassed by: jdumars

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[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-ci-robot]

@andyzhangx: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-unit	6920141	link	/test pull-kubernetes-unit

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 53694, 53919). If you want to cherry-pick this change to another branch, please follow the instructions here.