Summary:
Original commit: 9b4a2b1fd4af20b83100b80ad5fa55018bd3a626/D40173
`load_pem_private_key` doesn't support OpenSSH private keys.
This causes private key parsing to fail.
Use `load_ssh_private_key` to parse keys as well.

Test Plan: Manually verified parsing OpenSSH keys generated with `ssh-keygen -t rsa -b 2048 -f ~/.ssh/test_key`.

Reviewers: nsingh, anabaria

Reviewed By: anabaria

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D40268