[RFE] Change config options to be mutable

SIGHUP support for ironic [1][2] will allow the ironic-api and ironic-conductor services to reload mutable configuration options. To start with, these configuration options are mutable:

  * ``[DEFAULT]/debug``
  * ``[DEFAULT]/log_config_append``
  * ``[DEFAULT]/pin_release_version``

This RFE will make most of the ironic configuration options mutable. The work can be done in one patch or a series of patches, but we should try to land them all in a short amount of time, and in the same named release so as not to confuse operators as to which options were made mutable and in which release.

The work involves:

  * deciding which options make sense to be mutable (they should be listed here in a comment, we probably shouldn't approve this w/o an idea of what those options are
  * making sure those option values are not cached

[1] RFE/bug: https://bugs.launchpad.net/ironic/+bug/1585595
[2] specification: http://specs.openstack.org/openstack/ironic-specs/specs/approved/sighup_on_ironic_services.html