work around bogus linkspeed reported by tuntap

The tuntap driver unconditionally reports its devices as ethernet
devices with speed 10MBit. Being a virtual device it will happily
transfer way higher rates and trigger error states in
check_iftraffic_nrpe. This commit disables autodetection for tun
devices.
Merge branch 'workaround-tuntap'

Conflicts:
	check_iftraffic_nrpe.py (master introduced default_values)