Specifying a container on the public index fails due to / in container name

```
core@core-01 ~ $ cat .toolboxrc 
TOOLBOX_DOCKER_IMAGE=coreos/apache
TOOLBOX_USER=root
core@core-01 ~ $ /usr/bin/toolbox
Pulling repository coreos/apache
87026dcb0044: Download complete 
511136ea3c5a: Download complete 
6170bb7b0ad1: Download complete 
9cd978db300e: Download complete 
2014/04/15 19:00:24 Error: Invalid container name (core-coreos/apache), only [a-zA-Z0-9_.-] are allowed
2014/04/15 19:00:24 Error: No such container: core-coreos/apache
tar: This does not look like a tar archive
tar: Exiting with failure status due to previous errors
Error: No such container: core-coreos/apache
2014/04/15 19:00:24 Error: failed to remove one or more containers
touch: cannot touch '/var/lib/toolbox/core-coreos/apache/etc/os-release': No such file or directory
Directory /var/lib/toolbox/core-coreos/apache lacks the binary to execute or doesn't look like a binary tree. Refusing.