Add Wants=network-online.target to amazon-ssm-agent.service

[thimslugga]

Issue #, if available:

Description of changes:

Add Wants=network-online.target, see the following resource for systemd and proper usage of network-online.target.

https://systemd.io/networking/NETWORK_ONLINE

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[ziggythehamster]

See also: https://github.com/aws/amazon-ssm-agent/pull/280