Add a format_message method to the Exceptions

This patch adds a format_message method to the NovaException class that
will almost always do what a user wants, when they want to print out the
message of the exception.

Exceptions that are received through RPC are turned into an actual
exception classes by the deserialize_remote_exception function of the
nova.openstack.common.rpc.common module. This function then overrides
__str__ and __unicode__ methods of a new class to print the whole trace.
We still want to preserve this functionality (for logging) but do not
want to leak the trace back to the user in the HTTP response.

Based on the above, This patch attempts to establish a convention that
states: if you need to get the message of a NovaException subclass -
always use the format_message method. Only use str(e) or unicode(e) when
you suspect that they were overriden.

Initially this was attempted through adding a custom metaclass to the
NovaException class so that format_message cannot be overridden in
subclasses, however due to a bug in testtools - many tests were failing.
A patch was proposed to testtols to remedy this
(https://github.com/testing-cabal/testtools/pull/34) however until it is
merged, we cannot use metaclasses with our exception classes as they
cause tests to fail.

Change-Id: I0a399b9ab3227bad644b031c9b689fe0ce64586e
Merge "Add a format_message method to the Exceptions"