Summary:
yb-admin and yb-ts-cli can only specify the certs directory with certs_dir_name, but
certs_dir shows up in the help text and could also be used. This diff makes both of them valid, with
preference for certs_dir_name if set.
Jira: DB-17900

Test Plan:
Ran `./yb_build.sh --cxx-test integration-tests_external_mini_cluster_secure_test --gtest_filter ExternalMiniClusterSecureTest.YbTools` with `certs_dir` instead of `certs_dir_name` and it worked.
Test failed with a random flag name.

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: svc_phabricator, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D45974