Add dns-servers config option for upstream dns servers

Adds a dns-servers config option for specifying the forwarding
dns servers to be used by the dnsmasq services on the neutron
dhcp agent. This enables services using internal dns to also
specify the forwarding dns servers in order to resolve hosts
outside of the neutron network space.

Change-Id: I3cd608b1491a45f565d5147894b8285e638eeaa7
Implements: blueprint internal-dns
Closes-Bug: #1713721

config.yaml

@@ -166,6 +166,12 @@ options:
     description: |
       Comma-separated list of key=value config flags with the additional
       dhcp options for neutron dnsmasq.
+  dns-servers:
+    type: string
+    default:
+    description: |
+     A comma-separated list of DNS servers which will be used by dnsmasq as
+     forwarders.
   enable-l3-agent:
     type: boolean
     default: True

hooks/neutron_contexts.py

@@ -90,6 +90,7 @@ class NeutronGatewayContext(NeutronAPIContext):
             'debug': config('debug'),
             'verbose': config('verbose'),
             'instance_mtu': config('instance-mtu'),
+            'dns_servers': config('dns-servers'),
             'l2_population': api_settings['l2_population'],
             'enable_dvr': api_settings['enable_dvr'],
             'enable_l3ha': api_settings['enable_l3ha'],

templates/mitaka/dhcp_agent.ini

@@ -13,6 +13,10 @@ debug = {{ debug }}
 dnsmasq_config_file = /etc/neutron/dnsmasq.conf
 {% endif -%}
 
+{% if dns_servers -%}
+dnsmasq_dns_servers = {{ dns_servers }}
+{% endif -%}
+
 {% if dns_domain -%}
 dns_domain = {{ dns_domain }}
 # Per LP#1583769, dhcp_domain needs to be configured as well. Additional

unit_tests/test_neutron_contexts.py

@@ -162,6 +162,7 @@ class TestNeutronGatewayContext(CharmTestCase):
         self.test_config.set('instance-mtu', 1420)
         self.test_config.set('dnsmasq-flags', 'dhcp-userclass=set:ipxe,iPXE,'
                                               'dhcp-match=set:ipxe,175')
+        self.test_config.set('dns-servers', '8.8.8.8,4.4.4.4')
         self.test_config.set('vlan-ranges',
                              'physnet1:1000:2000 physnet2:2001:3000')
         self.test_config.set('flat-network-providers', 'physnet3 physnet4')
@@ -177,6 +178,7 @@ class TestNeutronGatewayContext(CharmTestCase):
             'shared_secret': 'testsecret',
             'enable_dvr': True,
             'enable_l3ha': True,
+            'dns_servers': '8.8.8.8,4.4.4.4',
             'dns_domain': 'openstack.example.',
             'local_ip': '10.5.0.1',
             'instance_mtu': 1420,
@@ -233,6 +235,7 @@ class TestNeutronGatewayContext(CharmTestCase):
             'shared_secret': 'testsecret',
             'enable_dvr': True,
             'enable_l3ha': True,
+            'dns_servers': None,
             'dns_domain': 'openstack.example.',
             'local_ip': '192.168.20.2',
             'instance_mtu': 1420,