Description of problem:
Currently only two commands are available for package-groups
-install
-remove

I think we should add command to list installed package-groups on selected host

 # hammer -u admin -p changeme host package-group --help
Usage:
    hammer host package-group [OPTIONS] SUBCOMMAND [ARG] ...

Parameters:
 SUBCOMMAND                    subcommand
 [ARG] ...                     subcommand arguments

Subcommands:
 install                       Install packages remotely
 remove                        Uninstall packages remotely

Options:
 -h, --help                    print help


Version-Release number of selected component (if applicable):
katello-2.5.0-3.nightly.el7.noarch
foreman-release-1.11.0-0.develop.201601211808gitd546fd0.el7.noarch
tfm-rubygem-hammer_cli_katello-0.0.20-1.el7.noarch
tfm-rubygem-hammer_cli-0.5.1-2.el7.noarch

How reproducible:
always

Steps to Reproduce:
1.hammer -u admin -p changeme host package-group --help
2.
3.

Actual results:
Missing sub-command to list installed package-groups on selected host.

Expected results:


Additional info: