This commit gets rid of the 'NONE' and 'INFO' severity levels for
deprecation issues.
'NONE' is unused and does not make much sense as a severity level.
'INFO' can be separated into two categories: Either 1) we can
definitively tell there will be a problem with the cluster/node/index
configuration that can be resolved prior to upgrade, in which case
the issue should be a WARNING, or 2) we can't, because any issues would
be at the application level, for which the user should review the
deprecation logs and/or response headers.
Benjamin Trent
lcawl
Gordon Brown
Przemyslaw Gomulka