fix ec2 instance creation with delete volume enabled (issue #18315)
- `ec2.query()` requires non-empty `param` argument
- replace `param`-less call to `ec2.query()` with `show_delvol_on_destroy()`
- TODO: remove `requesturl` as none of the callers of `_toggle_delvol()`
  make use of it
Merge pull request #18948 from walgitrus/fix-ec2-toggle-delvol

fix ec2 instance creation with delete volume enabled (issue #18315)