Customize shell; remove stale LocalSettings.php

- If auto-generated LocalSettings.php is present, but the database it specifies
  is not, assume it is left over from a previous VM instance and re-run
  install.php.
- Correct 'mediawiki' in .gitignore to '/mediawiki/', so it doesn't ignore
  files in puppet/modules/mediawiki.
- Add color.sh and phpsh.sh to /etc/profile.d: the former specifies a colorful
  prompt should be used, and the latter makes phpsh always execute with
  /vagrant/mediawiki as its cwd.
- Consume '--debug' flag from the command line; if it is present, pass it on to
  Puppet.
- Specify 'Ubuntu_64' os_type to VirtualBox.

Change-Id: Ic989d6b9ff56419ec77b4c6d5d2c8dcf5341a499