Rework database uri parsing/setting to include user supplied database

Building on #1 , mysql-proxy sends the incorrect database information is sent to slurmdbd.

Setting mysql-proxy config:
```bash
secret_id=$(juju add-secret mysql-proxy-db-uri db-uri="mysql://user:pass@host:port/slurm_acct_db_production")
```

Produces:

```bash
$ sudo cat /etc/slurm/slurmdbd.conf
dbdhost=*****
dbdaddr=*****
authalttypes=auth/jwt
authaltparameters=jwt_key=/etc/slurm/jwt_hs256.key
dbdport=6819
authtype=auth/slurm
slurmuser=slurm
plugindir=/usr/lib/x86_64-linux-gnu/slurm-wlm
pidfile=/var/run/slurmdbd/slurmdbd.pid
logfile=/var/log/slurm/slurmdbd.log
storagetype=accounting_storage/mysql
storageuser=user
storagepass=pass
storageloc=slurm_acct_db
storagehost=host
storageport=port
```

We need to account for setting the correct database information supplied by the user.

Start by locating the database URI parsing and slurmdbd.conf generation entry points; the issue does not name specific files or tests. Trace the supplied URI through configuration and verify that slurmdbd receives slurm_acct_db_production, with the other credentials and connection fields preserved.

Written by the indexing model from the issue text.