Change schedule priority of daemon/server in config

The goal is to modify schedule priority and I/O scheduling class and
priority of daemon/server via configuration.
Setting is optional, default keeps current behaviour.

Use case:
Prioritize object-server to object-auditor, because all user's requests
needed to be served in peak hours and audit could wait.

Co-Authored-By: Clay Gerrard <clay.gerrard@gmail.com>
DocImpact
Change-Id: I1018a18f4706daabdb84574ffd9a58d831e68396

etc/object-expirer.conf-sample

@@ -25,6 +25,16 @@
 # log_statsd_default_sample_rate = 1.0
 # log_statsd_sample_rate_factor = 1.0
 # log_statsd_metric_prefix =
+#
+# You can set scheduling priority of processes. Niceness values range from -20
+# (most favorable to the process) to 19 (least favorable to the process).
+# nice_priority =
+# You can set I/O scheduling class and priority of processes. I/O niceness
+# class values are realtime, best-effort and idle. I/O niceness
+# priority is a number which goes from 0 to 7. The higher the value, the lower
+# the I/O priority of the process. Work only with ionice_class.
+# ionice_class =
+# ionice_priority =
 
 [object-expirer]
 # interval = 300
@@ -51,6 +61,16 @@
 # queue.
 # reclaim_age = 604800
 # recon_cache_path = /var/cache/swift
+#
+# You can set scheduling priority of processes. Niceness values range from -20
+# (most favorable to the process) to 19 (least favorable to the process).
+# nice_priority =
+# You can set I/O scheduling class and priority of processes. I/O niceness
+# class values are realtime, best-effort and idle. I/O niceness
+# priority is a number which goes from 0 to 7. The higher the value, the lower
+# the I/O priority of the process. Work only with ionice_class.
+# ionice_class =
+# ionice_priority =
 
 [pipeline:main]
 pipeline = catch_errors proxy-logging cache proxy-server