re-enable AIX password setting

the AIX class uses a unsafe shell for setting the user password (containing a pipe in the command). This patch adopts to the new behavior of module_utils/basic.py (since somewhere around 1.7).

besides it changes the qoutes for the echo command from double to single, because password-hashes contain $-signs and one would not have this variables expanded.
Merge pull request #524 from zitterbacke/patch-1

re-enable AIX password setting