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
Adds caches for image and flavor in compute agent

Currently to obtain image and flavor data,
compute agent always calls nova api, even for the same id of image/flavor.
In order to reduce the number of nova-client call, caches for image and flavor
are introduced.

The caches only live in a local scope of _with_flavor_and_image: created every
single polling interval and thrown away at the end of the interval.
Although the caches only live in a short time, they are effective because same
flavors are used many times in one compute-node in typical use cases.

Change-Id: I63b6a59f06be4911f39073e34cfc3a2a7b832c51
Closes-bug: #1293337
Merge "Adds caches for image and flavor in compute agent"
Merge "Use TYPE_GAUGE rather than TYPE_CUMULATIVE"