add support for sections to config-flags option

[Edward Hope-Morley]

Some of our charms have a config-flags option that allows settings to be inserted into the [DEFAULT] section of Openstack config files but we now have an increased demand to allow manipulation of settings in other sections (many settings have been moved away from the default section and into their own since this feature was added).

For completeness it is necessary to state that this is not a recommended way to configure services. Doing so can easily have adverse affects e.g. by clashing with configs applied by the charms and perhaps breaking upgrades since local changes are not tested by upstream. Nevertheless, we provide the ability to inject settings in order to allow users to trial config alterations and we encourage them to propose their changes back to the upstream charms if they are generally applicable.