Expose skipImmediately in Schedule Template

[pniederloehner]

Describe the problem/challenge you have
After changing Velero chart configuration a scheduled backup is triggered immediately

Describe the solution you'd like
Velero schedule CRD supports option "skipImmediately". This should be configurable in helm template as well.

Anything else you would like to add:
If accepted, I would like to create a PR for it