Description of problem:

If mock is run on a system with selinux enabled, and the yum package is not installed, mockbuild/plugins/selinux.py will crash as it attempts to load /usr/share/yum-cli/cli.py

INFO: mock.py version 1.4.2 starting (python version = 3.6.2)...
Start: init plugins
INFO: selinux enabled
ERROR: No module named 'cli'
Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 934, in <module>
    main()
  File "/usr/lib/python3.6/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 703, in main
    buildroot = Buildroot(config_opts, uidManager, state, plugins, bootstrap_buildroot)
  File "/usr/lib/python3.6/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.6/site-packages/mockbuild/buildroot.py", line 70, in __init__
    self.plugins.init_plugins(self)
  File "/usr/lib/python3.6/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.6/site-packages/mockbuild/plugin.py", line 65, in init_plugins
    module.init(self, self.plugin_conf["{0}_opts".format(plugin)], buildroot)
  File "/usr/lib/python3.6/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.6/site-packages/mockbuild/plugins/selinux.py", line 27, in init
    SELinux(plugins, conf, buildroot)
  File "/usr/lib/python3.6/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.6/site-packages/mockbuild/plugins/selinux.py", line 56, in __init__
    if self._selinuxYumIsSetoptSupported():
  File "/usr/lib/python3.6/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.6/site-packages/mockbuild/plugins/selinux.py", line 111, in _selinuxYumIsSetoptSupported
    import cli
ModuleNotFoundError: No module named 'cli'



Version-Release number of selected component (if applicable):
mock-1.4.2-1.fc26.noarch

How reproducible:
Always


Steps to Reproduce:
1. Setup system as noted above
2. mock --old-chroot -r fedora-26-x86_64 --buildsrpm --spec <spec> --sources $PWD