doc: fix hyphens instead of underscores for 'os*' conf options

The ceilometer documentation, on the configuration page,
list some options with hyphens (e.g. 'os-tenant-id') which are unknown options:
underscores are required instead (e.g.: 'os_tenant_id').

Fixes: bug #1177002
Change-Id: Iea9c53f628df0b3bed06206e9b2241ed949131fa
Cherry picked from https://review.openstack.org/#/c/30725/