Allow specifying a listen IP

Fixes: Bug #1183314

- Update api options in ceilometer/api
- Create 'api' option group
- Read options in bin/ceilometer-api
- Update sample config
- Updated tests

Change-Id: Ic76d95f262ac9b0675cf37698c9bcdf65aff9821
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
Merge "doc: fix hyphens instead of underscores for 'os*' conf options"
Merge "Allow specifying a listen IP"