Ubuntu 20.04. CI/CD Pipeline as GitHub Action

[udosson]

This PR solves #1678 and #1679 and provides the GHA workflow for the CI and the CD pipeline of Indy-Node. Also, this PR contains an update of the ubuntu-20.04-upgrade feature branch with the most recent commits of master. The PR contains:

• CI workflow based on master branch and Indy-Plenum
• CD workflow based on master branch and Indy-Plenum
• Fixed linting issues with newer version of flake8 (flake8==3.8.4) and python 3.8
• updated the version of Indy-SDK to python3-indy==1.15.0-dev-1625, the same version as used in Indy-Plenum which contains the freeze leger support.
• adjusted runner.py to be compatible with the newer version of pytest (pytest>=6.2.2)
• install_requires in setup.py is based on indy-plenum==1.13.0.dev135 which was built and published from the ubuntu-20.04-upgrade branch of Indy-Plenum in this GHA run
• copied the uploading structure of the debian file to jfrog artifactory from master branch

A successful run of the workflow can be found at: https://github.com/udosson/indy-plenum/actions/runs/1186783441

Thanks to @WadeBarnes for all your support!

[sovbot]

Can one of the admins verify this patch?

[WadeBarnes]

@udosson, Looking good. Last thing to do is to squash/rebase the CI/CD updates onto this commit:

So the merge from the main branch stays intact and all of the CI/CD updates get included as one commit on top of that commit.

[udosson]

@WadeBarnes, thanks for your review. I squashed all commits from 6f2d3b6 on into one commit.