services/horizon: Make stellar core database URL optional if captive core is enabled.

[abuiles]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Disable stellar core database URL validation if captive core is enabled.

Why

Fix #3129

Known limitations

[TODO or N/A]

[bartekn]

LGTM! I think we should simplify ingest.System init in the future by passing a LedgerBackend implementation to it.

[2opremio]

Have you made sure that nothing breaks when this is nil? (unfortunately successful integration tests don't help at this point because they run without captive core).

[abuiles]

@2opremio yes, I check the different code paths and we are now only using this value if CaptiveCore is disabled.