slapos.cookbook:zero-knowledge.read has this limitation that because it
reads the secret file on __init__, ie. before any other parts are
installed, the value cannot be read the first time.
On first slapos node instance run, the generated turnserver-config has
an empty secret, it's only after the second execution that the secret is
generated. What happens is:
 - first run:
   - read-secret __init__ cannot find the file and does not override
     secret
   - turnserver-config uses the default ${read-secret:secret} (empty
     string)
   - gen-secret install creates the secret file
 - second run
   - read-secret __init__ finds the file and override secret with the
     value read from the file.
   - turnserver-config uses the overriden ${read-secret:secret} (good
     secret)

This was not a problem before, because some frontend promise were
always failing, so we processed this partition several time, but now it
is succesfully processed the first time.

To workadound this, we prevent turnserver from starting if secret is
empty, then promise will fail, instance will be processed a second time
and then turnserver will run with a secret.