Replace dashes in link name with underscores

This PR sanitizes environmental variables derived from link names. Since link names are allowed to contain dashes, the environmental variables would be invalid.

Closes https://github.com/dotcloud/docker/issues/5418

Docker-DCO-1.1-Signed-off-by: Jonathan Camp <jonathan@irondojo.com> (github: kung-foo)
You could probably do this and the command below in one line