Fix --output-file CLI argument

This commit fixes it so that the file in not overwritten each time a
minion returns information. Fixes #8205.
Remove existing output file if present

This saves the use of a filehandle by simply removing the file instead
of truncating it. when salt.output.display_output comes along to append
to the file, it will be created if it does not already exist.
Merge pull request #8258 from terminalmage/issue8205

Fix --output-file CLI argument