bash completion doesn't respect VAGRANT_HOME

When VAGRANT_HOME is set to a different path than $HOME/.vagrant.d, bash
completions regarding box names fails with an error like:

"$ vagrant init <TAB> find: `/home/mert/.vagrant.d/boxes': No such file or directory"

This commit makes completion.sh respect VAGRANT_HOME if it is set. It will continue to use $HOME/.vagrant.d otherwise.