Use TYPE_GAUGE rather than TYPE_CUMULATIVE

For hardware network bandwidth metering, it uses TYPE_CUMULATIVE, but
the oid "1.3.6.1.2.1.2.2.1.5"(in file ceilometer/hardware/inspector/snmp.py)
it walks in net-snmp.org is representing ifSpeed and the type is GAUGE.

Cumulative definition: Increasing over time, but when i run
`ceilometer sample-list --meter hardware.network.bandwidth.bytes`
the output is not increasing over time.

reference:
http://docs.openstack.org/developer/ceilometer/measurements.html
http://www.net-snmp.org/docs/mibs/interfaces.html

Change-Id: Ic8a8280695c25db103fe4e4592fe00622728655d
Closes-Bug: #1326723
Merge "Use TYPE_GAUGE rather than TYPE_CUMULATIVE"