Allow for direct Host assignment for already registered Push mode Agents

[Dark4Codrutz]

Is your feature request related to a problem? Please describe.
The problem arises when the Host object is created after an Agent has been already registered.
The only available way to assign an existing Agent to a new Host object is by going through the same new agent configuration form even if the agent is already configured and registered, e.g. in Push Mode.

Describe the solution you'd like
At least for Agents already registered in Push Mode there should be a way to simply assign it to a Host object by skipping the config.ini generation steps.

For example, from the Agent overview Push Mode list users are presented with an Assign Host button.
But upon clicking it we are presented with the same New Agent Configuration form.

My suggestion is to use that button to instead just make the user select/create new Host for the already registered Agent.

Describe alternatives you've considered
As a workaround the user has to fill in bogus server address and api key values in Configure Agent step just to force his way to the Select Agent and Create Service steps.

EDIT: typos