Update usage of 'ip' to handle more return codes
Update usage of 'ip' to handle more return codes

Make sure that ip raises an exception.ProcessExecutionError unless
'ip' exits with a 0, 2, or 254.

0 = command executed correctly
2/254 = command has already been executed.

Change-Id: I55dd63ca7fc22bc133ab5fe722df4141c332a819
Signed-off-by: Chuck Short <753a8074f04ffd62a8fff7516e3ae689be8e35ad@canonical.com>

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