Cleanup some initialization/flag parsing in rekor-server.

[dlorenc]

This is in preparation for supporting multiple logIDs (for sharding).

Signed-off-by: Dan Lorenc [email protected]

[dlorenc]

Up next:

• support starting the server with multiple log IDs, instead of just one.
  • this will probably look like a config file containing a map of index ranges to log ids.
  • only one can be "active"
• support parsing the UUID during GET calls to determine which log to query (with defaulting for the 0 case here)

[bobcallaway]

lgtm