Rename 'registry_tool' to 'nomulus'

This changes everything with external visibility beyond the codebase
(i.e. the name of the compiled binary and the documentation that refers
to it). It does not change a lot of things internal to the codebase,
i.e. the "RegistryTool" class didn't change its name. We can rename that
in a subsequent CL if we want to.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135022087

docs/configuration.md

@@ -5,7 +5,7 @@ working registry system up and running. Broadly speaking, configuration works in
 two ways -- globally, for the entire sytem, and per-TLD. Global configuration is
 managed by editing code and deploying a new version, whereas per-TLD
 configuration is data that lives in Datastore in `Registry` entities, and is
-updated by running `registry_tool` commands without having to deploy a new
+updated by running `nomulus` commands without having to deploy a new
 version.
 
 ## Environments
@@ -113,9 +113,9 @@ configuration. They contain any kind of configuration that is specific to a TLD,
 such as the create/renew price of a domain name, the pricing engine
 implementation, the DNS writer implementation, whether escrow exports are
 enabled, the default currency, the reserved label lists, and more. The
-`update_tld` command in `registry_tool` is used to set all of these options. See
-the "Registry tool" documentation for more information, as well as the
-command-line help for the `update_tld` command. Unlike global configuration
+`nomulus update_tld` command is used to set all of these options. See
+the [admin tool documentation](./admin-tool.md) for more information, as well as
+the command-line help for the `update_tld` command. Unlike global configuration
 above, per-TLD configuration options are stored as data in the running system,
 and thus do not require code pushes to update.