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)
It actually may make more sense to just use `strings.Replace` instead of a regex