Queued experiment fails due to missing metadata

[denkv]

Description

When the controller starts an already queued experiment after startup, it can fail with Couldn't find image name for benchmark.

Reproducability

Steps to reproduce the behavior:

1. Start an experiment
2. Restart the controller during the experiment
3. Make sure it cannot prepare the benchmark and system data too fast
4. See error

Expected behavior

The experiment should not fail just because of the controller being not ready.

Possible solutions

• wait until the initial metadata retrieval is done before starting any experiments
• store the full benchmark and system data in the experiment queue
• Cache the list of known benchmarks and systems #549