CML v0.18.15

• Add cml runner launch --cloud-image (#1302)
• Add commit hash to watermark (#1309)
• Rename --token to --driver-token (#1272)
• Fix cml runner idleness detection logic (#1299)
• Fix missing await and increase polling delay (#1300)
• Fix rate limit issues in parallel workflows (#1303)
• Bump version to v0.18.15 (#1312)