Update 'SIGHUP on ironic services'
Update 'SIGHUP on ironic services'

This updates the spec for SIGHUP support to:
1. provide a link to the RFE/bug for the work to make most of the
   ironic configuration options mutable
2. mention that mutable option values cannot be cached forever

This is a follow up to 2298f9895fff42c7dbb9f504715e692954db2243.

Change-Id: Ib78310e394ed03c63764080081e9e10cecfda036
Partial-Bug: #1585595

openstack/ironic-specs