Fix agent_id with string type in API samples files for os-agents v2

The agent_id should be integer for agent create and index. But in
the api sample file it is string type. It's because the api sample
tests provide agent_id with string type in fake data. This patch
correct the api sample files and tests.

For agent update, it use agent_id that passed from url, make the
response use string for agent_id. We can't fix this problem for
back-compatibility. This will be fix in the future after v3 API
expose by micro-version

Change-Id: I262b4b26c94dba003e80bda2f38d2e985ef9f220
Partial-Bug: #1333494
Merge "Fix agent_id with string type in API samples files for os-agents v2"