Move central agent code to the polling agent module

That's the preparation for further compute and central agents
merge

Partially-Implements-Blueprint: merge-compute-central-agents

Change-Id: If5f151ab7d30c9711929f6dcade553fdca09ab67
Merge Central and Compute agents to *polling agent*

Implements-Blueprint: merge-compute-central-agents

Change-Id: I0e8c38ba624d0f9c88582a8b2d7180e808d262a7
Fix docs to suit merged compute/central agents concept

DocImpact: Fix agents-related docs to describe polling agent
Related-Blueprint: merge-compute-central-agents

Change-Id: If377ac49bf75c0698f4af6be021e8354240f4e42
Merge "Move central agent code to the polling agent module"
Merge "Merge Central and Compute agents to *polling agent*"
Merge "Fix docs to suit merged compute/central agents concept"