Allow database prepare command to use environment variables.

[ghunteranderson]

The files /makedb/dbs/pg.js, makedb/dbs/mysql.js and /config/connections.js allow the user to pass the standard database connection environment variables into the prepare command so long as a connection URL is not set. However, if -u <url> is not passed into start.sh, then the kong.js invocation becomes ./bin/kong.js prepare --adapter postgres --uri. The argument parser then interprets this as argv.uri=true and the process tries to connect with connection URL "true". The changes in start.sh attempt to address this.

Moving on to kong.js, we omit setting process.env.DB_URI at all if argv.uri is empty. This prevents process.env.DB_URI from becoming string "undefined" and causing the same problem as string "true".

Finally, this PR removes the validation requiring a connection URL. I believe this is currently ignored if ran via startup.sh as argv.uri was either true or a nonempty string.

[ghunteranderson]

I believe this PR may address #323. All the @pantsel would need to do is add the standard environment variables DB_HOST, DB_USER, DB_PASSWORD, etc and omit the -u <url> argument.

For me, I have passwords that may not be url safe. The encodeURI function doesn't encode the password and uri.parse(str) can't correctly parse if ":", "@" and "/" are not encoded. Using the environment variables are a great alternative and were my first preference.