Enhancement/improve validation calls

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Unit test passes: ?
Phpspec specification passes: yes
Checkstyle issues: no
ChangeLog updated: no
Fixes the following issue: #166 

MagentoSoapClientParameters is now a singleton registry and have a valid (or not) state.
This state is usefull to know if the validation has to be called or not.