Store joshua agent image tag in FDB

[sfc-gh-qhoang]

Motivation Currently when we update either joshua-scaler or joshua-agent, we must manually update BOTH of their corresponding image tags.

@sfc-gh-kmakino proposes to store the agent image tag in coordinating FDB itself so the scaler can query and use up-to-date agent images without us having to go through the deployment to update. This will save us time when we only need to update the agent image tag.

What's in this PR?

• Changes to implement the ability to get/set agent image tag in joshua FDB

Testing

• I am not sure how to test these changes at the moment and looking for suggestions.

Related PR

• A PR needed to extract out $AGENT_TAG in the agent yaml config file.

[sfc-gh-qhoang]

We should make this feature optional. e.g. if you use latest tag for the agent, you don't need this feature.

we discussed offline but basically since scaler_agent.sh replaces env variables with envsubst, there will be no behavior change if there does not exist an $AGENT_TAG env var in the yaml config file.