WHAT is this pull request doing?

Makes it possible to configure default user and password. Takes a hashed value for password.

Also adds api endpoint /api/auth/hash_password/:password and lavinmqctl command lavinmqctl hash_password password to hash passwords.

Also changes guest-only-loopback to default-user-only-loopback, since the default user no longer has to be guest.

Fixes #877

Configuration

Values can be configured by environment variables, in the config file or by command line arguments.
Env

LAVINMQ_DEFAULT_USER=USER
LAVINMQ_DEFAULT_PASSWORD=PASSWORD

Config file

[main]
default_user=USER
default_password=PASSWORD

Command line
--default-user=USER --default-password=PASSWORD

HOW can this pull request be tested?

Run specs.