Fix InvalidContentType can't be raised because of error in constructor

InvalidContentType exception class is defined two times:
in quantum.openstack.common.exception and quantum.common.exceptions
with two different signatures.
A lot of code like wsgi.Serializer.serialize() imports one of them but
attempts to use it with the signature of another one, which ends with
TypeError exception.

This change fixes the bug by removing the second definition of
InvalidContentType class and leaving only one which works correctly
with all code. Also adds unit tests for exceptions to prevent this and similar bugs.

Change-Id: I5d932d75ad184a0a6c6419190f2940bd47b7504e
Fixes: bug #1104090
Merge "Fix InvalidContentType can't be raised because of error in constructor"