HDDS-3093. Allow forced overwrite of local file

[adoroszlai]

What changes were proposed in this pull request?

Add -f (--force) flag for ozone sh key get, to allow force overwrite of local file. This is useful for testing:

• download the same key multiple times without having to manually delete the target
• download to /dev/null (which already exists), only to verify existence and checksum of the key

Hash cannot be calculated for /dev/null.

This PR is based on top of #775, so that one should be merged first. The only change here is 2ca1fe2.

https://issues.apache.org/jira/browse/HDDS-3093

How was this patch tested?

$ ozone freon ockg -n 1 -p test
...

$ ozone sh key get /vol1/bucket1/test/0 /tmp/asdf

$ ozone sh key get /vol1/bucket1/test/0 /tmp/asdf
/tmp/asdf exists. Download would overwrite an existing file. Aborting.

$ ozone sh key get --force /vol1/bucket1/test/0 /tmp/asdf

$ ozone sh --verbose key get --force /vol1/bucket1/test/0 /tmp/asdf
Volume Name : vol1
Bucket Name : bucket1
Key Name : test/0
Downloaded file hash : 528e4f96633a453d18ee01fdb5d7c536

$ ozone sh --verbose key get --force /vol1/bucket1/test/0 /dev/null
Volume Name : vol1
Bucket Name : bucket1
Key Name : test/0

Added smoke test for overwrite case.

[xiaoyuyao]

LGTM overall, comments added inline.

[adoroszlai]

Thanks @xiaoyuyao for the review.

[dineshchitlangia]

+1 @adoroszlai thanks for working on this, @xiaoyuyao thanks for reviewing.

[adoroszlai]

Thanks @dineshchitlangia for reviewing and merging it.