[SIGMON-158] Separate sets of requirements for Nxcals Spark 2 and Nxcals Spark 3

Context

• task: https://its.cern.ch/jira/browse/SIGMON-158 (cleanup after https://its.cern.ch/jira/browse/SIGMON-133)

What was done

• venv_xxx dirs on SWAN contain separate dirs for Nxcals Spark 2 and Nxcals Spark 3 requirements
• lhcsmapi is installed in venv_xxx/commons
• SWAN setup scripts check the version of python and add appropriate requirements dir to the PYTHONPATH

TODO

• I created a task to version the scripts used by the experts: https://its.cern.ch/jira/browse/SIGMON-159

QA

• no tests - CI

Review objectives

• assessment of code functionality
• assessment of code quality
• identifying potential refactorings / logic common to other projects
• knowledge sharing