state: implement Machine.SetMachineBlockDevices
state: add test for block devices
Merge pull request #1139 from axw/state-machine-setblockdevices

state: implement Machine.SetMachineBlockDevices

Next step is to enable the diskmanager in machine
agents, at which point block devices on the machine
will be periodically recorded in state.
Limit collect-metrics hook to fire only on charms that have metrics declared.
Merge pull request #1142 from tasdomas/filter-collect-metrics-hook

Limit collect-metrics hook to fire only on charms that have metrics declared.

Currently when in debug-hooks mode, the collect-metrics hook will be executed periodically (even when the charm has no metrics-collect hook script) - this is annoying to charm developers.