fix(BA-725): Increase Backend.AI Kernel's app startup timeout (#3672)

[MintCat98]

resolves #3672 (BA-725)

Increased the kernel-timeout value in kernel/base.py:

• prev: 5.0s
• curr: 30.0s

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations