Fix inventory file and upgrade documentation

[mateigrigore]

Summary:

This PR will improve the setup process for local development of the mongodb-kubernetes-operator. The documentation contained outdated and misleading information. Also some problems with building images and deploying the operator were encountered because of using a local registry. This

The PR adds:

• Documentation changes to improve the local setup process
• Changes to the Makefile:
  • Change the name of the parameter to look for the agent image in the config.json file to agent_image
  • Add --ignore-not-found flag to uninstall-crd target because of errors encountered when calling this during the make deploy command where no crd was installed yet
  • Add $(IMG_BUILD_ARGS) to all image building targets to allow the --insecure flag to be passed as a paramter
• Changes to the inventory.yaml file:
  • Removed default values for agent_distro and tools_distro
  • Changed the values for agent_distro and tools_distro in buildargs to be extracted from the params
• Changes to the pipeline.py file:
  • Allow for a new argument insecure to be passed

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