Refactor config keys missionId and spacecraft.id to better reflect purpose and usage

[rmseng]

Perform the following:

• Change config key missionId to product.sclkScetFile.missionId
  • Make the key required only if SCLK-SCET product creation is enabled
• Change config key spacecraft.id to product.sclkScetFile.spacecraftId
  • Make the key required only if SCLK-SCET product creation is enabled