[v3] Better Config File Support

Follow up to PR #287.

Add better support to config support. This PR implements the card:

https://trello.com/c/WsDMg7RP/161-initial-support-for-avocado-config-files-server-and-client

As a matter of fact, avocado has support for config files, it was only not very visible, and we did not create the config file automatically in the user's config dir. This patch changes that, and adds a new plugin for config values configuration and visualization, that supersedes the `datadir` plugin.

Changes from v1:
 * Read from system wide, then override with local config values instead of only reading local config values if available.
 * Added support to avocado.conf.d.

Changes from v2:
 * Create an empty config file at the user's config location, instead of copying system wide values.
 * Remove the `datadir` plugin, as it has redundant functionality.