Favor MSF_DATABASE_CONFIG for paths['config/database']

[MSP-10848](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&selectedIssue=MSP-10848)

This replaces https://github.com/rapid7/metasploit-framework/pull/3578

# Verification Steps

## msfconsole

### With MSF_DATABASE_CONFIG

- [x] `export MSF_DATABASE_CONFIG=${pro}/ui/config/database.yml` (`${pro}` should be set to the location of your pro repository)
- [x] `./msfconsole`
- [x] `db_status`
- [x] VERIFY msfconsole is connected to the pro database and not the metasploit-framework database (**NOTE: you should be using a different database for metasploit-framework and pro**) 
```
[*] postgresql connected to metasploit_pro_production
```
- [x] `exit`
- [x] `unset MSF_DATABASE_CONFIG`

### Without MSF_DATABASE_CONFIG

- [x] `./msfconsole`
- [x] `db_status`
- [x] VERIFY msfconsole is connected to the framework database defined in `~/.msf4/database.yml` (**NOTE: you should be using a different database for metasploit-framework and pro**) 
```
[*] postgresql connected to metasploit_framework_private_development
```
- [x] `exit`

## specs

- [x] `rake spec`
- [x] VERIFY no failures