Global environment ignores files starting with dot

The _list_environment_files() method was modified to ignore all the
files, in the heat.common.config.environment_dir, with names that starts
with a dot. This method uses glob.glob instead of os.listdir.

Change-Id: Ida84472adef771a7310277fdb549d358877fe739
Closes-Bug: #1272114
Merge "Global environment ignores files starting with dot"