Implement pg_autoctl do azure commands.

[DimCitus]

The following commands allow to easily deploy a testing or QA environment
for pg_auto_failover using Azure VMs, which looks more like what typical
users and customers are using than our default make cluster approach where
all the nodes are running on localhost.

In particular we can test system integration and packages based
provisioning, so that's better for end-to-end QA.

Here's the list of new commands provided:

pg_autoctl do azure
+ provision  provision azure resources for a pg_auto_failover demo
+ tmux       Run a tmux session with an Azure setup for QA/testing
+ show       show azure resources for a pg_auto_failover demo
  deploy     Deploy a pg_autoctl VMs, given by name
  create     Create an azure QA environment
  drop       Drop an azure QA environment: resource group, network, VMs
  ls         List resources in a given azure region
  ssh        Runs ssh -l ha-admin <public ip address> for a given VM name
  sync       Rsync pg_auto_failover sources on all the target region VMs

pg_autoctl do azure provision
  region  Provision an azure region: resource group, network, VMs
  nodes   Provision our pre-created VM with pg_autoctl Postgres nodes

pg_autoctl do azure tmux
  session  Create or attach a tmux session for the created Azure VMs
  kill     Kill an existing tmux session for Azure VMs

pg_autoctl do azure show
  ips    Show public and private IP addresses for selected VMs
  state  Connect to the monitor node to show the current state

And there's the new top-level commands:

make azcluster
make azdrop

Contrary to make cluster when exiting the tmux session the resources are not dropped, and it's possible to connect again to the same tmux session with either make azcluster or pg_autoctl do azure tmux session.

[JelteF]

I skimmed the code in this PR and it looked reasonable. Since this is essentially just dev helpers I think it's fine to merge this without me doing a very thorough review. I ran make azcluster and make azdrop. They worked, but there's some pg_hba lines that we probably want to add. I think it's fine to do that in a later PR though.