Refactor install_plugins() in install_plugins.py

This is an unfinished refactoring sub-task coming from the [Add Pylint as a python static code analyzer #298](https://github.com/bitovi/bitops/pull/298).

The `install_plugins()` function in `install_plugins.py` is failing pylint check on:
> scripts/plugins/install_plugins.py:17:0: R0914: Too many local variables (22/15) (too-many-locals)
> scripts/plugins/install_plugins.py:17:0: R0915: Too many statements (59/50) (too-many-statements)

The function has almost 200 lines and pylint with multiple warnings is a great indicator that it needs to be restructured and divided into smaller logical pieces. For instance:
* Extract it into a new `Plugin` class perhaps with another layer `PluginInstaller ` (note: `PluginDeployer` #328) with multiple methods, so the interface would be `Plugin.install()` that we can call while iterating over each plugin in the sequence.
* Then another class `PluginConfig` would handle plugin config parsing and processing.

The pylint check for this function is silenced for now, don't forget to unsilence it when the refactoring is done.