feat(agents): improve agent config

[lattejed]

Agent config should be improved to speed up deployments. Currently most config changes have to go through a PR->deploy cycle. Configs should be updatable after deployment

• Agent config should be simplified with sensible defaults
• All necessary settings should be overridable with env vars

This touches or umbrellas some existing issues:

#149
#159
#81

Context / more info:

#148 (56c4807)
#135
#40
#39
#34

[lattejed]

More config info here: https://docs.nomad.xyz/dev/agents/agent-development.html

[luketchang]

More config info here: https://docs.nomad.xyz/dev/agents/agent-development.html

These are unfortunately stale :( working on updating for external operators

[lattejed]

#171 covers only part of this, so I'm going to keep this open