Use os_password_x pattern for compute and baremetal commands

Migrate compute.py and baremetal.py to use the same credential
handling pattern as other manage commands (image, server, volume,
etc.). This eliminates the dependency on a pre-existing secure.yml
file and instead loads passwords dynamically from the OpenStack
secrets.yml using the os_password_{cloud} keys.

Changes:
- Replace get_cloud_connection() with setup_cloud_environment()
- Add --cloud parameter to all commands (default: admin)
- Use try/finally pattern to ensure cleanup of temporary files
- Replace get_cloud_project() calls with conn.identity.get_project()

Affected commands:
- osism manage compute (enable/disable/list/evacuate/migrate/
  migration list/start/stop)
- osism manage baremetal (list/deploy/dump/undeploy/ping/burnin/
  clean/provide/maintenance set|unset/power on|off/delete)

AI-assisted: Claude Code

Signed-off-by: Christian Berendt <berendt@osism.tech>

Author: berendt