Serve Bellatrix preset in BN API (#3425)

## Issue Addressed

Resolves #3388
Resolves #2638

## Proposed Changes

- Return the `BellatrixPreset` on `/eth/v1/config/spec` by default.
- Allow users to opt out of this by providing `--http-spec-fork=altair` (unless there's a Bellatrix fork epoch set).
- Add the Altair constants from #2638 and make serving the constants non-optional (the `http-disable-legacy-spec` flag is deprecated).
- Modify the VC to only read the `Config` and not to log extra fields. This prevents it from having to muck around parsing the `ConfigAndPreset` fields it doesn't need.

## Additional Info

This change is backwards-compatible for the VC and the BN, but is marked as a breaking change for the removal of `--http-disable-legacy-spec`.

I tried making `Config` a `superstruct` too, but getting the automatic decoding to work was a huge pain and was going to require a lot of hacks, so I gave up in favour of keeping the default-based approach we have now.

beacon_node/src/cli.rs

@@ -229,8 +229,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("http-disable-legacy-spec")
                 .long("http-disable-legacy-spec")
-                .help("Disable serving of legacy data on the /config/spec endpoint. May be \
-                       disabled by default in a future release.")
+                .hidden(true)
+        )
+        .arg(
+            Arg::with_name("http-spec-fork")
+                .long("http-spec-fork")
+                .help("Serve the spec for a specific hard fork on /eth/v1/config/spec. It should \
+                       not be necessary to set this flag.")
+                .takes_value(true)
         )
         .arg(
             Arg::with_name("http-enable-tls")