Support for loading custom data from Azure metadata service

[niusmallnan]

For Azure, there are two ways to VM provisioning: WALA and cloud-init.

Currently we are just using WALA(https://github.com/Azure/WALinuxAgent), it is very simple and can handle SSH keys and hostname. The custom data passed in by the user is just placed in /var/lib/waagent, and ROS will ignore it.

We should handle custom data if the user does specify it.

https://docs.microsoft.com/en-us/azure/virtual-machines/linux/instance-metadata-service

[Jason-ZW]

create VM with custom data

AZURE_ROS_SSH_PUBLIC_KEY="xxxxxx"
az vm create -g rancheros \
         -n rancheros \
         --admin-username rancher  \
         --image rancheros --size Standard_A1 \
         --ssh-key-value "$AZURE_ROS_SSH_PUBLIC_KEY" \
         --custom-data /tmp/cloud-config.txt

cloud-config

#cloud-config
hostname: rancheros1

cloud-config-script

#!/bin/sh
echo "aaa" > /home/rancher/aaa.txt

[niusmallnan]

We have already asked the Azure Team, and currently we can't set custom data on the UI.
The only way is passing the custom data by CLI.

[rootwuj]

Tested with rancher/os:v1.5.2-rc1 from May 27
This version supports cloud-init configuration VM, test with comment.
Verified fixed.