expanduser on each component of plug-in paths

ansible.constants was calling expanduser (by way of shell_expand_path)
on the entire configured value for the library and *_plugins
configuration values, but these values have always been interpreted as
multiple directories separated by os.pathsep.  Thus, if you supplied
multiple directories for one of these values, typically only the first
(at least on *nix) would have e.g. "~" expanded to HOME.

Now PluginLoader does expansion on each individual path in each of
these variables.
Merge pull request #3316 from dsedivec/devel

expanduser on each component of plug-in paths