Fixes #1513 - distinguish between RHEL 7 Server and Workstation

When facts are received we parse them and create/assign the operating
system of a host accordingly. We didn't correctly detect the right RHEL
7 distribution and considered every RHEL 7 as Server. For people having
both variants in their infrastructure, this caused problems.

With this patch, RHEL 7 Workstations is recognized as
RedHat_Workstation, the OS can't contain spaces. We don't have other
attribute to distinguish these distributions and it's not worth
introducing it as a separate attribute, since this concept was replaced
by system purpose in RHEL 8.