Description of problem:
When we add an already existing hook to a migplan, and we click on "Cancel" button instead of "Add hook", the next time that we want to add a hook the UI will show a wrong screen.

Version-Release number of selected component (if applicable):
MTC 1.5.0

How reproducible:
Alwasy

Steps to Reproduce:
1. In MTC main screen, click on "Hooks" and create a hook
2. Create a migration plan and go to "Add hook" screen
3. Click on "Add hook" and select the hook created in step 1

"Hook name" and "hook definition" fields will be disabled.

4. Click on "Cancel"
5. Click again on "Add hook"


Actual results:
The hook created in step 1 is the default value now and the "hook name" and "hook definition" fields are disabled already but they are empty. If we try to create the hook with those values the form will complain about those fields being mandatory.

In case we want to use this default hook, we need to unselect it and select it again.

Expected results:
If we want to keep the last selected hook as the default value after a cancel operation, all fields need to be filled in taking this into account.

If we want to present all the fields in the form with an empty value after a cancel operation, then the default value for the "Add an existing hook or create a new one *" field should be  "Create a new hook" and do not disable any field.

Additional info: