services/horizon/cmd: Fix prepare range bug

[tamirms]

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

Fixes #3580

The horizon db reingest range command failed because it requires that the top level horizon ingest flag to be true.
In other words, horizon --ingest db reingest range works but horizon db reingest range responds with the following error:

2021/05/03 12:49:43 Invalid config: one or more captive core params passed (--stellar-core-binary-path or --captive-core-config-append-path) but --ingest not set. 

This PR fixes this bug by making all ingestion subcommands set ingest to true implicitly.

This PR also fixes the captive core config validation by only requiring the config file to be present when ingesting in online mode. It is not necessary to have a captive core config file when ingesting bounded ledger ranges.

[paulbellamy]

LGTM, but the code scanning warning does look like it might be a thing.