Increase max scheduling attempts to 10

At high scale, such as 64 nodes with 13TB ram and 2600 cores, nova
seems to struggle when scheduling 100+ VMs at the same time.  The
issue is unrelated to the database, as the error printed indicates
the max_scheduling_attempts have been reached.  Increase that value
to something more fitting of a 100 node cluster.

Change-Id: I8982d77c7c66db8f7c95b9fd73f58ceb66dbd723
Closes-Bug: #1563664

ansible/roles/nova/templates/nova.conf.j2

@@ -24,6 +24,8 @@ ec2_listen_port = {{ nova_api_ec2_port }}
 use_neutron = True
 firewall_driver = nova.virt.firewall.NoopFirewallDriver
 
+scheduler_max_attempts = 10
+
 {% if neutron_plugin_agent == "openvswitch" %}
 linuxnet_interface_driver = nova.network.linux_net.LinuxOVSInterfaceDriver
 {% elif neutron_plugin_agent == "linuxbridge" %}