install: Don't require the network by default

The `Wants=network-online.target` predates the osmet work which enabled
the now default fully offline install flow.

It also predates the addition of fetch retries. So even in an online
flow, now that we retry HTTP requests indefinitely, we don't really
need this.

Let's follow best practices and stop pulling in `network-online.target`.

We still need to keep pulling in `NetworkManager.service`
though. It's enabled by default in `multi-user.target` but not
`coreos-installer-post.target` (which is what we boot to in an automated
install).

NetworkManager is capable of handling offline environments just fine and
won't block the install just because a connection isn't available (yet
or ever).

Related: coreos/fedora-coreos-config#1088
Related: #565 (comment)
Related: https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/
Closes: https://github.com/coreos/coreos-installer/issues/1334

docs/release-notes.md

@@ -11,6 +11,7 @@ Major changes:
 
 Minor changes:
 
+- install: Don't require the network by default
 
 Internal changes:
 

systemd/coreos-installer.service

@@ -1,8 +1,8 @@
 [Unit]
 Description=CoreOS Installer
 Before=coreos-installer.target
-After=network-online.target
-Wants=network-online.target
+After=NetworkManager.service
+Wants=NetworkManager.service
 # Until we retry HTTP requests let's wait here until
 # systemd-resolved comes up if enabled.
 # https://github.com/coreos/coreos-installer/issues/283