Fixed wrong flavor validation

* Nova has designed feature that deleted flavor could be shown by its id.
* Nova flavor list command doesn't show deleted flavors, so moved validation to check flavors in nova.flavors.list()
* Updated appropriate unit tests

Change-Id: I252c155f1d1f31bdacc4f2fc2e6f9da47ee05a38
Closes-Bug: #1257019
Drop os.common.exceptions

The os.common.exceptions lib was deprecated in oslo and all usages should be
removed.

We're only using os.common.exceptions.Error:

class Error(Exception):
    def __init__(self, message=None):
            super(Error, self).__init__(message)

and we have our own base exception class with all needed stuff
savanna.utils.exceptions.SavannaException.

Change-Id: Ib4a1601e4b71bb38d293d457afaffa1c979b8ffb
Closes-Bug: #1208734
Merge "Drop os.common.exceptions"
Merge "Fixed wrong flavor validation"