services/horizon/cmd: Fix db-url flag parsing in horizon db commands

[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

Close #3191

The horizon db migrate and horizon db init commands share the db-url command line parameter with the horizon service flags defined in services/horizon/cmd/root.go. The horizon service flags are also initialized in root.go:

func init() {
	err := flags.Init(rootCmd)
	if err != nil {
		stdLog.Fatal(err.Error())
	}
}

To parse the db-url param in the migrate and init commands, we need to call flag.Require() and flag.SetValue().

The reason why this code wasn't working before is because we were calling on Require() and SetValue() on dbURLConfigOption which was never initialized against any of the db commands or the root command.

Known limitations

[N/A]

[bartekn]

Please add CHANGELOG. This will be 1.11.1.

[bartekn]

Suggested change

	log.Fatal("could not find db-url flag")
	log.Fatalf("could not find %s flag", name)