[keyserver] Specify host to mysqldump

Summary:
Right now it's basically hardcoded to use localhost. We have this config in DBConfig, so we should use it. This will make the backup work if the database is not local.

Depends on D4097

Test Plan:

1. I checked mysqldump --help | grep host on both my local macOS as well as on the prod server and it looks like it's definitely -h
2. I tested the backup cronjob by modifying the time config
3. I tested localhost to make sure it works as an alias to 127.0.0.1

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4098