Add tls_handshake_timeout paramter to override the default 10 seconds

[ywei2017]

• A short explanation of the proposed change:
  Add tls_handshake_timeout so it can be configured

• An explanation of the use cases your change solves:
  When the diego_cells go through vSphere vMotion, there is a stun time (like the VM is frozen). When that happens, new gorouter connections to the diego_cell will timeout after 10 seconds. The parameter will allow us to shorten the timeout, so gorouter would attempt another diego_cell, mitigating the long wait (or 499 errors) on the clients.

• Instructions to functionally test the behavior change using operator interfaces (BOSH manifest, logs, curl, and metrics)
  Inspect /var/vcap/jobs/gorouter/config/gorouter.yml. When the parameter is not provided, it should say "tls_handshake_timeout: 10s"; or whatever value is provided by the operator.

• Expected result after the change
  Gorouter clients see faster response time during vMotion, or fewer 499 erros

• Current result before the change
  Gorouter clients experience timeouts.

• Links to any other associated PRs
  N/A

• I have viewed signed and have submitted the Contributor License Agreement

• I have made this pull request to the develop branch

• I have run all the unit tests using scripts/run-unit-tests-in-docker

• (Optional) I have run Routing Acceptance Tests and Routing Smoke Tests on bosh lite

• (Optional) I have run CF Acceptance Tests on bosh lite

[cf-gitbot]

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The labels on this github issue will be updated when the story is started.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Yansheng Wei (2925f50, 79821a9)

[geofffranks]

Thanks @ywei2017 !