Fixes #38264 - prevent duplicate entries

[sbernhard]

What are the changes introduced in this pull request?

Prevent to bind to the on ContentLoad multiple times which would lead to duplicate entries in the Lifecycle Environment Selection on the CreateHost page.

Considerations taken when implementing this change?

The window variable can be checked on loading the page if it does already exist

What are the testing steps for this pull request?

• Use the create host page
• Select the Hostgroup which uses ContentSource and all Katello related stuff
• Unselect (x) the Content Source
• Select the same Content Source
• Try to pick another Hostgroup and Unselect/Select the Content Source for another test
  At all stages, the Lifcecycle Environment Selection should not have duplicate entries.