(#7137) Get rid of spurious info messages in useradd

Usage of the useradd provider was leading to spurious log messages of
this form:

info: /User[nigel]: Provider useradd does not support features
manages_aix_lam; not managing attribute ia_load_module

This was due to the ia_load_module parameter requiring manages_aix_lam
and additionally having a defaultto value of "compat."

Paired-with: Matt Robinson <matt@puppetlabs.com>