Improve configuration files structure

[Ana06]

Details

Proposed changes to improve the configuration structure:

• Split config.xml into packages.xml (for the packages) and config.xml (post installation step inside the tags apps, services, path-items, registry-items, and custom-items) as it is done in Commando-VM. Note that the installer is already using these two files are we are duplicating the config.xml file in our installer. Splitting the configuration will provide a better overview of the configuration and hopefully make it easier for users to configure. It would also allow to configure the packages and keep the default post installations steps. This implies adding an argument to our installer to provide an extra custom file.
• Move all the configuration files (packages.xml, and config.xml, and CustomStartLayout.xml) to a default-config folder. This requires modifying the links in the installer. This folder will make it clearer for users which is the default configuration and what can be configured.

@mandiant/flare-vm, @HuskyHacks opinions?