Fix source recipe looking up URL with double slash

Introduced in 503926c7.
Remove 'racktables' from install_dir.

'html' in install_dir path is specific to CentOS.
Remove 'omnibus' reference in .kitchen.yml

This was a workaround for 'vagrant-berkshelf' conflicting with
'vagrant-omnibus'.
Change `install_dir` to extend apache docroot.

Tarball extraction now goes to a versioned directory within the install
dir. The version directory is then symlinked to 'current' under
`install_dir`.

This now makes the source deployment idempotent.