Log that authentication cannot be enabled if there is no admin account created

[ryanwalder]

If you enable auth in the config without creating a new admin user then restart the service there is no mention of this in the logs.

I know this is kind of expected behaviour, having it logged on startup would be good though something like:

[WARN] Unable to enable Authentication, no Admin users exist.

[beckettsean]

Related to #2193 and #2278

[gunnaraasen]

For 0.9.2, when authentication is turned on and no users exist, only the CREATE USER [user] WITH PASSWORD [user] WITH ALL PRIVILEGES command can be executed (see PR #3244). If one or more users exist, authorization is enforced as usual.

Once PR #3427 is merged, the logs will note whether authentication is enabled or disabled.