metrics-net: Skip if /sys/class/net/:iface_path is a regular file

If the host has a bond interface, there is
/sys/class/net/bonding_masters, which is a regular file.
In this case, metrics-net.rb fails to run.
Merge pull request #372 from maoe/fix-bonding-error

metrics-net: Skip if /sys/class/net/:iface_path is a regular file