Avoid IPv4 in the Network Underlay (IPv6 all the way!)

[smklein]

At the time of writing, our SMF manifests use IPv4 addresses by default:

omicron/smf/sled-agent/manifest.xml

Lines 44 to 46 in 8757ec5

	<propval name='bootstrap_addr' type='astring' value='127.0.0.1:12346' />
	<propval name='server_addr' type='astring' value='127.0.0.1:12345' />
	<propval name='nexus_addr' type='astring' value='127.0.0.1:12221' />

These should transition to IPv6 addresses to align with the expected addressing on the system.

Related to: #332

[bnaecker]

This is related to #889