Refactor to use imcl only

For your consideration.

* I found it easier to install using `imcl` rather than baking out an XML response file. For basic purposes, I found this sufficient. (http://www-01.ibm.com/support/knowledgecenter/SSDV2W_1.8.0/com.ibm.cic.commandline.doc/topics/c_imcl_container.html)
* Set `base_dir` to `/opt/IBM/InstallationManager` since that's what is recommended in the docs.
* Use a `scratch_dir` where we extract the IIM zip file to, because you can't install into the same `base_dir` as you've extracted into
* Set the user's shell to `/bin/sh` instead of bash, since AIX doesn't have bash by default.