[BUG] WALinuxAgent service should start after the cloud-config service

Currently (On Canonical UbuntuServer 18.04-LTS) the WALinuxAgent service and cloud-init run in parallel on start-up causing issues with contention of the package repository.

I believe that WALinuxAgent should start _after_ the cloud-config service to ensure that:
- Packages that extensions may depend on can be installed by cloud-init; (EG: https://github.com/microsoft/azure-pipelines-agent/issues/2866 for an example)
- Updates or packages installed by WALinuxAgent, or by Extensions, do not contend for the distribution's package manager, causing locks and failures

**Distro and WALinuxAgent details:**
- Distro and Version: [e.g. Ubuntu 18.04 latest image]
- WALinuxAgent version
```
WALinuxAgent-2.2.45 running on ubuntu 18.04
Python: 3.6.9
Goal state agent: 2.2.48.1
```

**Additional context**
My current situation when starting an Ubuntu 18.04 VM, with the Log Analytics extension is that:
1. WALinuxAgent starts, installs the Log Analytics agent (through extension) then;
2. Shortly after (~2 seconds or so) cloud-init starts with the following configuration:
```yaml
package_upgrade: true
package_reboot_if_required: true
```
3. WALinuxAgent or Log Analytics agent has the package repository lock, and causes cloud-init to fail

My current solution is to modify the service behaviour by adding this override to `/etc/systemd/system/walinuxagent.service.d/override.conf` from within `bootcmd` using cloud-init:
```
[Unit]
After=cloud-config.service
```

Start by inspecting the WALinuxAgent systemd service and the cloud-config.service ordering on Ubuntu 18.04. Reproduce the startup scenario with cloud-init package_upgrade enabled and the Log Analytics extension. Done means WALinuxAgent starts after cloud-config and the package manager no longer experiences the described lock contention.

Written by the indexing model from the issue text.