Summary:
It could happen that remote bootstrap happens over existing tablet.
For instance because of crash in the middle of previous remote bootstrap.
In this case moving vector index directory fails, since destination directory already exists.

Fixed by deleting existing directory in this scenario.

Also changed external mini cluster logic to start tablet servers in parallel, which decreased test execution time by 20 seconds.

**Upgrade/Rollback safety:**
Added fields that are used only in tests.
Jira: DB-16610

Test Plan: PgVectorIndexITest.CrashAfterRBSDownload

Reviewers: arybochkin

Reviewed By: arybochkin

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D43980