Refactor install_plugins() in install_plugins.py

[arm4b]

This is an unfinished refactoring sub-task coming from the Add Pylint as a python static code analyzer #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.