HAproxy/heat_api: increase timeout to 10m

HAproxy/heat_api: increase timeout to 10m

Default timeout is 2min but it doesn't reflect the rpc_response_timeout
value that we set in THT and instack-undercloud, which is 600 (10 min).

In some cases (in low-memory environments), Heat needs more than 2
minutes to reply to the client, when deploying the overcloud.
It makes sense to increase the timeout to the value of rpc_timeout to
give a chance to Heat to reply to the client, otherwise HAproxy will
kill the connection and send 504 to the client.

Depends-On: I9669d40d86d762101734704fcef153e360767690
Change-Id: I32c71fe7930c8798d306046d6933e4b20c22740c
Related-Bug: 1666072