removes project_id/tenant_id from v3 api urls

Adds a new APIMapper for V3 that doesn't prefix every
url with the project_id/tenant_id.

Partially implements blueprint v3-api-remove-project-id.

Change-Id: I547a94e2db0dd967852b3cb8532dd0adb11e693b
Merge "removes project_id/tenant_id from v3 api urls"