Description of problem:

    imgbase base --remove=<BASE> doe not remove child layers.

Version-Release number of selected component (if applicable):
    imgbased-0.8.11-0.1.el7ev.noarch

How reproducible:
    100%

Steps to Reproduce:
    1. imgbase base --remove=<BASE>

Actual results:
Child layer is not removed and other commands fails due to this inconsistency:

     # imgbase --debug base --remove=rhvh-4.0-0.20161116.0

     # imgbase base --latest
            Traceback (most recent call last):
            File "/usr/lib64/python2.7/runpy.py", line 162, in _run_module_as_main
                "__main__", fname, loader, pkg_name)
            File "/usr/lib64/python2.7/runpy.py", line 72, in _run_code
                exec code in run_globals
            File "/usr/lib/python2.7/site-packages/imgbased/__main__.py", line 51, in <module>
                CliApplication()
            File "/usr/lib/python2.7/site-packages/imgbased/__init__.py", line 82, in CliApplication
                app.hooks.emit("post-arg-parse", args)
            File "/usr/lib/python2.7/site-packages/imgbased/hooks.py", line 120, in emit
                cb(self.context, *args)
            File "/usr/lib/python2.7/site-packages/imgbased/plugins/core.py", line 137, in post_argparse
                print(app.imgbase.latest_base())
            File "/usr/lib/python2.7/site-packages/imgbased/imgbase.py", line 322, in latest_base
                return self.naming.last_base()
            File "/usr/lib/python2.7/site-packages/imgbased/naming.py", line 62, in last_base
                bases = self.bases()
            File "/usr/lib/python2.7/site-packages/imgbased/naming.py", line 50, in bases
                bases = sorted(self.tree())
            File "/usr/lib/python2.7/site-packages/imgbased/naming.py", line 224, in tree
                bases[img.base.nvr].layers.append(img)
            KeyError: <NVR rhvh-4.0-0.20161116.0 />


Expected results:
    The base is removed including its layers