Fix ValueError in ip_lib.IpRouteCommand.get_gateway()

As metric is not necessarily the 5th word of the gateway line, the
method should search the string 'metric' in the line and pick the next
word as the metric value.

Change-Id: I2663ddbae82f80b912b364c07f9ab92c5b90b718
Closes-Bug: #1267790
Merge "Fix ValueError in ip_lib.IpRouteCommand.get_gateway()"