fix bug where list-units ignores units of container with a dot in the name

this is related to https://github.com/openshift/geard/issues/173 and solves a bug in `gear list-units` where a containers installed like this:

`gear install openshift/busybox-http-app app.4` wouldn't be listed in `gear list-units -a` because it has a dot in the name.

I moved the container identifier pattern to a constant that can be references in other places to avoid this situation in the future.