system groups - allow user to change env/view for selected systems

Prior to this commit, we added to the UI the ability for
a user to 'edit' the environment and content view associated with
all systems in a system group.  This commit will extend that so
that if the user selects a few systems from the group, they
can edit only those systems.
system groups - add test for handling edit on selected systems
Merge pull request #1868 from bbuckingham/fork-system_groups

system groups - allow user to change env/view for selected systems
fixing rubygem(foreman-katello-engine) requires
Merge pull request #1878 from jlsherrill/scl_fix

fixing rubygem(foreman-katello-engine) requires
System groups: allow users to update systems via CLI