Fix API Key flyout double submit (elastic#167468)

## Summary

Closes elastic#163314


## Fixes

- Removed an extra EuiPortal that was not needed as EuiFlyout adds a
Portal
- Inverted control of the form by wrapping the flyout content in the
form. Prevents multiple submits by using traditional form controls and
button type as submit.


## Release Notes:

- Fixes issue with multiple API keys being created if the form is
submitted using the enter key fired multiple times in quick succession.
elastic#163314

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Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>