Obsolete attempts at determining the full path to the desired
'easy_install' executable were still left behind and need to be removed
for the customizable 'executable' task argument to correctly work:
'_get_easy_install' should take care of all the functionality related to
determining the correct 'easy_install' executable to use.
Merge pull request #4786 from SalesSeek/find-correct-easy-install

Fix determination of correct, possibly customized, 'easy_install' executable