[omdb] Find internal DNS servers via /etc/resolv.conf

[jgallagher]

There are three changes in this PR:

1. If not given an explicit DNS server address, omdb now uses the system resolver instead of assuming a hardcoded rack subnet.
2. When sled-agent reconfigures the switch zone after getting an underlay address from RSS, it also configures and enables the network/dns/install SMF service to populate /etc/resolv.conf.
3. When sled-agent starts the switch zone on a cold boot after rack setup, it includes network/dns/install in the site manifest.

I tested changes 1 and 2 in a4x2, and things look good:

root@g0:~# grep 'enabled DNS' $(svcs -L sled-agent) | looker
22:10:59.677Z INFO SledAgent (ServiceManager): enabled DNS install service with new nameservers
    file = sled-agent/src/services.rs:4018
    nameservers = [fd00:17:1:1::1, fd00:17:1:2::1, fd00:17:1:3::1]

root@oxz_switch:~# cat /etc/resolv.conf
nameserver fd00:17:1:1::1
nameserver fd00:17:1:2::1
nameserver fd00:17:1:3::1
root@oxz_switch:~# omdb nexus blueprints list
note: Nexus URL not specified.  Will pick one from DNS.
note: using DNS from system config (typically /etc/resolv.conf)
note: (if this is not right, use --dns-server to specify an alternate DNS server)
note: using Nexus URL http://[fd00:17:1:d01::5]:12232
...

I'll need a racklette to test change 3; I'll wait until #9653 lands to do so. (Or I'll rebase on top of it to test early next week, if it hasn't landed yet.)

Fixes #9606.