[SPIKE] Use python 3 across all distributions

[atsikham]

Is your feature request related to a problem? Please describe.
Currently we use an outdated python2 version that is not supported. Moreover, python versions across supported OS distributions are different. As a result some ansible modules cannot be used as it's not possible to install necessary dist packages. Probably it will cause other issues in future.
Example: python-cryptography required for openssl_* ansible modules cannot be installed on ubuntu 18.04:

The following packages have unmet dependencies:
 python-cryptography : Depends: python-asn1crypto but it is not installable
                       Depends: python-cffi-backend-api-min (<= 9729) but it is not installable
                       Depends: python-cffi-backend-api-max (>= 9729) but it is not installable
                       Depends: python-enum34 but it is not installable
                       Depends: python-idna (>= 2.1) but it is not installable
                       Depends: python-ipaddress but it is not installable
E: Unable to correct problems, you have held broken packages.

Describe the solution you'd like
Python3 is used on all target hosts for Ansible.

Describe alternatives you've considered
No

Additional context
Need to verify python dependencies and test that after switching all/main tasks work correctly.

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DoD checklist

• Reader is able to understand the results of spike
• The results of the spike are presented in a table (to show simply what are compared or researched parameters) / not applicable
• Each value / cell in the results table is described more deeply below
• Demo of the spike (automated as much as possible)
• Design doc updated

[atsikham]

The question is what kind of tests should be performed after python version upgrade.

[seriva]

I think we can review and close this after we are done with RedHat upgrade.

[to-bar]

Python 3.6 is present on RHEL 8 out of the box (as /usr/libexec/platform-python). We also install python3 package (Python 3.6.8).
This task can be closed after #1332 is done.