Documentation: action deployment needs mistral re-install

[do3meli]

as per https://docs.openstack.org/mistral/latest/contributor/creating_custom_action.html you have to reinstall mistral if you are deploying a custom action. this is absolutely impractical if you just want to release a new version of your action which is changing entry points or adding new additional actions. there must be a mechanism that allows you to just install your new pip without reinstalling mistral.

[Dougal Matthews (d0ugal)]

The documentation here isn't clear. The custom action needs to be reinstalled each time it is changed. The Mistral executors then need to be restarted (which isn't ideal).

We should update the documentation to better explain this and hopefully over time remove the need to restart Mistral.