Cleaning up exception handling

bug 1094035

Change-Id: I91e134a3632e6b3314fe90d25e2df780aaba54d1
minor improvements to nova/tests/test_metadata.py

There are 2 changes here:
 a.) previously we checked for existance of 'user-data' in
     a response, when it clearly would not have been there as
     the key name is 'user_data'.
 b.) Insist that top level keys in the openstack metadata do not have '-'.
     This is just a consistency thing.  Its very easy for someone to
     unintentionally add a key named 'my-field'.  We want to enforce
     keys to be named consistently using '_' rather than -.
     For reference, current keys are:
       files uuid availability_zone hostname meta public_keys name

Change-Id: Ia73050c5313b3524dd1d341528cc5f916e603c64
Merge "minor improvements to nova/tests/test_metadata.py"
Merge "Cleaning up exception handling"