Refactor the dev setup scripts to avoid duplication

Summary:
I recently added a third script (setup_lfs_server.sh) on top of the existing two, and that introduced even more copy and pasta; reviewers flagged that.
Duplication also makes it hard to add new common functionality (e.g. using configerator vs local files).

This diff introduces two more functions that let us cut down individual scripts by ~60%. The price for that is some complexity in the functions: consuming and filtering command-line flags pretty much requires arrays; reading and saving configuration requires references.
I tried to make future maintenance easier by adding good comments and unit tests.

Differential Revision: D54945927

fbshipit-source-id: c58a5b7b6110bc7f789d331d609fb802f08be6f0

eden/mononoke/facebook/scripts/TARGETS

@@ -1,3 +1,4 @@
+load("@fbcode_macros//build_defs:custom_unittest.bzl", "custom_unittest")
 load("@fbcode_macros//build_defs:native_rules.bzl", "buck_filegroup")
 load("@fbsource//tools/build_defs:glob_defs.bzl", "glob")
 load("//antlir/fbpkg:fbpkg.bzl", "fbpkg")
@@ -28,3 +29,11 @@ fbpkg.builder(
         "scripts": ":scripts",
     },
 )
+
+custom_unittest(
+    name = "setup_common_test",
+    command = [
+        "eden/mononoke/facebook/scripts/setup_common_test.sh",
+    ],
+    type = "simple",
+)