2.9.0

What's Changed

• Add shell completions by @bartekpacia in #780
• Allow mounting NBD disks by @fkorotkov in #786
• tart pull: retry VM pull with exponential backoff by @edigaryev in #788
• tart stop: throw RuntimeError.VMNotRunning consistently and use enumeration instead of strings by @edigaryev in #784
• Fixed Swift Warning by @fkorotkov in #787

Full Changelog: 2.8.1...2.9.0