Refactor all gRPC proxy code into a single class.

appengine/swarming/doc/Magic-Values.md

 # Magic Values
 
 Describes magic values on the swarming server
 
 ## Introduction
 
 There are a few "magic" values in the isolate and swarming server. Also some
 dimensions and state values have special meaning.
 
 
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 When a Swarming bot is running a task, the following environment variables are
 always set:
 
   - `SWARMING_HEADLESS=1` is always set.
   - `SWARMING_BOT_ID` is set to the bot id.
   - `SWARMING_TASK_ID` is set to the task id.
 
 The following environment variables may be set to alter bot behavior:
 
   - `SWARMING_EXTERNAL_BOT_SETUP=1` disables bot_config.setup_bot hook.
+  - `SWARMING_GRPC_PROXY=<url>` and `ISOLATED_GRPC_PROXY=<url>` override the
+    equivalent value in the bot config.
+  - `LUCI_GRPC_PROXY_VERBOSE` dumps out additional gRPC proxy information if set
+    to a truthy value (e.g. `1`).
+  - `LUCI_GRPC_PROXY_TLS_ROOTS=<file>` and points to a .crt file containing
+    certificate authorities. `LUCI_GRPC_PROXY_TLS_OVERRIDE=<name>` specifies the
+    name of the server in the certificate. These are useful for testing a gRPC
+    proxy running on localhost but with TLS enabled. Unlike the `*_GRPC_PROXY`
+    env vars, these are shared between Swarming and Isolated since they're only
+    used in the limited case when you need to override TLS. See
+    [/client/utils/grpc_proxy.py](../../../client/utils/grpc_proxy.py) for more
+    information.
 
 
 ### dimensions
 
   - `id`: must be a single value in the list, which also must be unique. It's
     what uniquely identify the bot.
   - `quarantined`: if present, it specifies the bot self-quarantined, as it
     found out it needs manual sysadmin assistance before being able to accept
     any task. An example is that it doesn't enough free disk space.
 
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     handshake. The server will ask the bot to restart if this configuration
     changes.
 
 ### tags
 
   - `source_revision`: if present, it specifies the SCM revision related to the
     task.  This allows the UI to link to the relevant revision.
   - `source_repo`: if present, it is a url to the hosted SCM related to the
     task, with a %s where the revision should be placed.  This allows the UI
     to link to the relevant revision.