Report server domain to other servers:

This commit introduces a new configuration option that server
operators can set. The value is communicated to other servers
and is also reported via the `server_info` API.

The value is meant to allow third-party applications or tools
to group servers together. For example, a tool that visualizes
the network's topology can group servers together.

Similar to the "Domain" field in validator manifests, an operator
can claim any domain. Prior to relying on the value returned, the
domain should be verified by retrieving the xrp-ledger.toml file
from the domain and looking for the server's public key in the
`nodes` array.

src/ripple/core/impl/Config.cpp

@@ -19,6 +19,7 @@
 
 #include <ripple/basics/FileUtilities.h>
 #include <ripple/basics/Log.h>
+#include <ripple/basics/StringUtilities.h>
 #include <ripple/basics/contract.h>
 #include <ripple/beast/core/LexicalCast.h>
 #include <ripple/core/Config.h>
@@ -496,6 +497,18 @@ Config::loadFromString(std::string const& fileContents)
             MAX_JOB_QUEUE_TX);
     }
 
+    if (getSingleSection(secConfig, SECTION_SERVER_DOMAIN, strTemp, j_))
+    {
+        if (!isProperlyFormedTomlDomain(strTemp))
+        {
+            Throw<std::runtime_error>(
+                "Invalid " SECTION_SERVER_DOMAIN
+                ": the domain name does not appear to meet the requirements.");
+        }
+
+        SERVER_DOMAIN = strTemp;
+    }
+
     if (getSingleSection(
             secConfig, SECTION_AMENDMENT_MAJORITY_TIME, strTemp, j_))
     {