Add scheduler config options for hold expiration

This adds max_hold_expiration and default_hold_expiration as
scheduler options.

max_hold_expiration sets the absolute maximum age, in seconds,
a node placed in the hold state will remain available. This
defaults to 0, which means there is no maximum.

default_hold_expiration sets the default value used if no value
is supplied. This defaults to max_hold_expiration.

Change-Id: Ia483ac664e0a2adcec9efb29d3d701f6d315ef3b

tests/fixtures/zuul-hold-expiration.conf

@@ -0,0 +1,38 @@
+[gearman]
+server=127.0.0.1
+
+[statsd]
+# note, use 127.0.0.1 rather than localhost to avoid getting ipv6
+# see: https://github.com/jsocol/pystatsd/issues/61
+server=127.0.0.1
+
+[scheduler]
+tenant_config=main.yaml
+relative_priority=true
+default_hold_expiration=1800
+max_hold_expiration=3600
+
+[merger]
+git_dir=/tmp/zuul-test/merger-git
+git_user_email=zuul@example.com
+git_user_name=zuul
+
+[executor]
+git_dir=/tmp/zuul-test/executor-git
+
+[connection gerrit]
+driver=gerrit
+server=review.example.com
+user=jenkins
+sshkey=fake_id_rsa_path
+
+[connection smtp]
+driver=smtp
+server=localhost
+port=25
+default_from=zuul@example.com
+default_to=you@example.com
+
+[web]
+static_cache_expiry=1200
+root=https://zuul.example.com/