Added SOA configuration for DNS settings. (#4714)
This will allow to fine TUNE SOA settings sent by Consul in DNS responses,
for instance to be able to control negative ttl.
Will fix: https://github.com/hashicorp/consul/issues/4713
# Example
Override all settings:
* min_ttl: 0 => 60s
* retry: 600 (10m) => 300s (5 minutes),
* expire: 86400 (24h) => 43200 (12h)
* refresh: 3600 (1h) => 1800 (30 minutes)
```
consul agent -dev -hcl 'dns_config={soa={min_ttl=60,retry=300,expire=43200,refresh=1800}}'
```
Result:
```
dig +multiline @localhost -p 8600 service.consul
; <<>> DiG 9.12.1 <<>> +multiline @localhost -p 8600 service.consul
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 36557
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;service.consul. IN A
;; AUTHORITY SECTION:
consul. 0 IN SOA ns.consul. hostmaster.consul. (
1537959133 ; serial
1800 ; refresh (30 minutes)
300 ; retry (5 minutes)
43200 ; expire (12 hours)
60 ; minimum (1 minute)
)
;; Query time: 4 msec
;; SERVER: 127.0.0.1#8600(127.0.0.1)
;; WHEN: Wed Sep 26 12:52:13 CEST 2018
;; MSG SIZE rcvd: 93
```
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