[Ipopt] Ipopt print_level option

[Brad Bell]

I need to suppress all the Ipopt output for my automated tests. In 
Ipopt-3.4.0 I was able to suppress all the Ipopt output by setting the 
print level to -2.

Using Ipopt-3.6.1 it appears that the lowest possible print level is 0 
and this does not seem to suppress the following message:

******************************************************************************
This program contains Ipopt, a library for large-scale nonlinear 
optimization.
 Ipopt is released as open source code under the Common Public License 
(CPL).
         For more information visit http://projects.coin-or.org/Ipopt
******************************************************************************

NOTE: You are using Ipopt by default with the MUMPS linear solver.
      Other linear solvers might be more efficient (see Ipopt 
documentation).