Change to use vcsrepo module and parameterize "revision" variable

This changes the phpmyadmin class to use the "vcsrepo" Puppet module instead of calling git directly. This has several advantages:

* vcsrepo won't do an unnecessary "git fetch" if it's already at the right revision, since it uses "ls-remote" first. This can save a lot of bandwidth on subsequent runs.
* vcsrepo automatically requires the "git" package.
* vcsrepo automatically creates the destination directory if it doesn't exist.

I also parameterized the "revision" variable so users can specify different versions of PHPMyAdmin and aren't limited to the latest stable version.