medusa offline install needs an update

[paayers]

Project board link

The offline install for Medusa is here:

https://github.com/thelastpickle/cassandra-medusa/blob/master/docs/Installation.md#offline-installation

The current RHEL/centos instructions do not work, but I’ve verified the following approach works:

install python3 and create a new virtual environment:
sudo python3 -m venv test

move into the created directory:
cd test

activate the new environment:
source bin/activate

next, upgrade pip/setuptools:
sudo bin/pip3 install --upgrade pip
sudo bin/pip3 install --upgrade pip setuptools

now, install medusa:
sudo bin/pip3 install cassandra-medusa

move out of the created directory:
cd ..

once the install is successful, create the requirements.txt file containing dependencies:
sudo test/bin/pip3 freeze > requirements.txt

create a new dir called medusa_requirements:
mkdir medusa_requirements

using the requirements.txt file, download all the included files into the new directory:
sudo test/bin/pip3 download -r requirements.txt -d medusa_requirements

now move the requirements.txt file into the medusa_requirements directory:
mv requirements.txt medusa_requirements/

now tar up the contents of the medusa_requirements directory:
sudo tar -zcf medusa_reqs.tar.gz medusa_requirements/
Now you should be able to transfer the medusa_reqs.tar.gz tarball to your env with no internet access.
Once transferred, unzip the files and install the libraries, for example:

tar -zxf medusa_reqs.tar.gz
pip3 install -r medusa_requirements/requirements.txt --no-index --find-links medusa_requirements

[paayers]

Jira for reference in case it's needed:
https://datastax.jira.com/browse/DOC-4071

[rzvoncek]

Relates to #555 .

[rzvoncek]

I've updated the offline / RHEL installation docs in #555.