Description of problem:
imgbase base --of-layer fails with exception when base is provided instead of a layer.

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

How reproducible:
100%

Steps to Reproduce:
1. base --of-layer <BASE>

Actual results:
imgbase base --of-layer rhvh-4.0-0.20170104.0
Base
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 139, in post_argparse
    print(str(app.imgbase.base_of_layer(args.of_layer)))
  File "/usr/lib/python2.7/site-packages/imgbased/imgbase.py", line 339, in base_of_layer
    return Image.from_nvr(layer).base
AttributeError: 'Base' object has no attribute 'base'


Expected results:
An error message is provided or the same base is reported back.

Additional info:
# imgbase base --of-layer rhvh-4.0-0.20170104.0+1
rhvh-4.0-0.20170104.0


# imgbase base --of-layer rhvh-4.0-0.20170104.0
Base
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 139, in post_argparse
    print(str(app.imgbase.base_of_layer(args.of_layer)))
  File "/usr/lib/python2.7/site-packages/imgbased/imgbase.py", line 339, in base_of_layer
    return Image.from_nvr(layer).base
AttributeError: 'Base' object has no attribute 'base'

Base class does not have a base object so when the base is detected instead of layer the exception is raised.