Formalize conventions for Exec resources

* lowercase_with_underscore
* Omit path, unless it's useful for some reason (for example, /bin/true to
  avoid confusion with boolean true).
* No spaces in name (unless the name is the command).
* Prefer a short, human-readable name to a command.

Change-Id: Ib52a6e77ff918903d466a7056aa0162dcf17fb4e
Only run mediawiki_reload provisioner if trigger exists

The provisioner itself checks for the presence of a TRIGGER file, but checking
in Vagrantfile spares us the 'Running provisioner mediawiki_reload...' line on
each invocation of 'vagrant provision'.

Change-Id: I8e2a293fce1df45e2d0b6384019d70cee1e6df85