Add GCP challenges

[benno001]

• Create challenge with secret in state
• Create challenge with pod exec
• disable links of certain challenge is not possible
• Update javacode to recognize environment and switch explanation
• Add explanations for GCP
• Have consistent AWS/GCP naming for challenge-explanations
• Create challenge 11 in GCP (Create challenge with overprivileged service account)

[bendehaan]

Addressed in #87