Summary:
Change WantedBy for node-agent systemd module so as the node-agent service starts on system reboot

```
yugabyte@ip-10-9-83-69:~$ systemctl --user list-units --type target --state active
  UNIT           LOAD   ACTIVE SUB    DESCRIPTION
  basic.target   loaded active active Basic System
  default.target loaded active active Main User Target
  paths.target   loaded active active Paths
  sockets.target loaded active active Sockets
  timers.target  loaded active active Timers
```

multi-user.target is never reached for user level systemd.

Test Plan:
Provisioned a node using YNP
Verified node-agent is running.
Rebooted the machine, verified that the node-agent is running post reboot

Reviewers: nsingh, anijhawan

Reviewed By: nsingh, anijhawan

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D41375