Run nodetool using results of find_erts_dir

Before this change, extended_bin.dtl assumes the release uses an
embedded erts release when calling nodetool, making any
nodetool-related functionality incompatible with the following
setting:

  {include_erts, false}

This change calls nodetool using the escript binary in the path
discovered in the find_erts_dir function.
Merge pull request #192 from timclassic/master

Run nodetool using results of find_erts_dir