Look for database.yml in installer location, too

Note that this only fixes the immediate crash in the installed environment by allowing msfconsole to find `database.yml`. We still have the bigger problem that if there is no `database.yml` or if the database is not listening, we can't even parse options, which is tracked by MSP-10905.

MSP-10848

# Pre-verification Steps
- [x] Merge https://github.com/rapid7/metasploit-framework/pull/3615 to fix blocking bug on this same ticket

# Verification
 - [ ] in an installer environment,
   - [ ] symlink this branch as /opt/metasploit/apps/pro/msf3
   - [ ] msfconsole
   - [ ] **Verify** No stack trace, msfconsole starts normally