Fix incorrect $::operatinsystem value assumption on Ubuntu hosts

The dell::openmanage class incorrectly assumes that Ubuntu operatingsystems
will return Debian, when the actually return Ubuntu.

This fix expands the case similar to the RedHat/Centos case where it will
correctly match (and load the required library) on a Ubuntu machine. As
it stands, the way teh current module is written, you have to explicitly
call dell::openmanage::debian on Ubuntu hosts.