The retry logic for VMPooler has two issues. 1) Waiting longer than
VMPooler takes to refill the pool to retry introduces excessive delays,
and 2) waiting for a deterministic amount of time means that processes
started at the same time stay in sync causing peak loads.
This change caps the wait time at 15 seconds plus a random wait time,
addressing both issues.
Previously, beaker-vmpooler defined its
connection preferences as string options, but
beaker provides defaults as symbols. This ends
with beaker saying that it doesn't support methods
that it does due to the mismatch. This will get
rid of those messages, as the provided methods
will be symbols, matching what beaker expects
