Add support for Azure virtual machine availability sets (VMAS)

[feiskyer]

Continue of #449.

This PR adds initial support for Azure virtual machine availability sets (VMAS). Significant changes include

• Add vmType parameters to indicate different virtual machine types. Candidate values are 'vmss' and 'standard'
• Add agent pools implementation (standard vmType) of NodeGroup
• Add more options for scaling agent pools
• Add more fake clients and unit tests

Special notes for reviewers:

• Only one node group supported yet
• Some functions (in azure_util.go) are copied from acs-engine. They are not ventured because of various vendor problems, e.g. i18n and go template. acs-engine should be adjusted before vendoring those functions
• Azure API calls may hit requests count limits which need to be addressed in further PRs

/cc @brendandburns @andyzhangx @jdumars

/assign @mwielgus @MaciekPytel

[feiskyer]

Update: Verified one node group scaling properly in following scenarios

• Cluster deployed via acs-engine without windows containers (standard vmType)
• Cluster deployed via acs-engine with windows containers (standard vmType)
• Cluster manually deployed on Azure VMSS (vmss vmType)

Also /cc @wbuchwalter (author of Kubernetes-acs-engine-autoscaler). Could you also take a look at this?

[mwielgus]

Is the Azure library also updated in the core Kuberentes?

[mwielgus]

Please fix the conflicts.

[feiskyer]

Is the Azure library also updated in the core Kuberentes?

@mwielgus Azure library version not updated in this PR, only adding a new sub-package.

[feiskyer]

Please fix the conflicts.

Rebased and conflicts resolved.

[mwielgus]

/lgtm

[feiskyer]

@mwielgus thanks