ADE: Add support for python3

Redhat recommends **python3** as the default on **RHEL 8.0** as per the following blog post:
https://developers.redhat.com/blog/2018/11/14/python-in-rhel-8/

```
$ waagent --version
WALinuxAgent-2.2.26 running on redhat 8.0
Python: 3.6.6
Goal state agent: 2.2.37

$ python --version
Python 3.6.6
```
*AzureDiskEncryptionForLinux* fails:
```
[stderr]
Expected Python 2.7, found Python 3.6.6
```

Start by locating the AzureDiskEncryptionForLinux code that checks for Python 2.7, then reproduce the failure on RHEL 8 with Python 3.6.6. Done means the extension supports the reported Python 3 environment without the “Expected Python 2.7” failure.

Written by the indexing model from the issue text.