Provide the first endpoint as default option for new plans

Problem/Motivation

Followup to #2914855: Improve UX of create Plan form where the deploy plan form was simplified for the user., however currently there is no endpoint selected when users create a new deploy plan, so the user has to expand the Show Deploy plan settings fieldset to set it or no endpoint will be associated with the plan.

Proposed resolution

Provide the first available endpoint as a default value.

Remaining tasks

Do it.

User interface changes

None.

API changes

None.

Data model changes

None.