Update "Exercising the Services Locally" doc

The sample config used to have several drivers enabled, but this was changed
to default to the 'pxe_ipmitool' production driver. The developer documentation
for "Exercising the Services Locally", has examples using the 'fake_ipmitool'
driver. It won't work since only the pxe_ipmitool driver is available. This adds
instructions for updating the config file to enable fake_ipmitool instead of
pxe_ipmitool.

Users sometimes encounter the 'NodeLocked' exception while trying out the
sample "ironic" commands, due to the periodic sync_power_state task locking
the node. Documentation was added with instructions on how to turn off this
background sync_power_state task via a config setting.

Change-Id: Id53db08df59d13ca7a52505d6b3c0931551f4bf4
Merge "Update "Exercising the Services Locally" doc"