Fix interface type check in nailgun agent

The regular expression used in ohai to determine interface type
doesn't work with interfaces containing more than one digit (e.g.
eth10). Consequently the "type" is returned as eth1 instead of eth
for all interfaces above eth9 and the agent skips them.

Ohai bug here: https://tickets.opscode.com/browse/OHAI-571

Change-Id: Ib302542255ec25bee43e4f012d66658219ec1710
Closes-bug: #1308036