Feature/msp 10230/snmp login


VERIFICATION STEPS
- [x] have a linux machine running snmpd with default settings
- [x] `msfconsole`
- [x] `use auxiliary/scanner/snmp/snmp_login`
- [x]  set RHOSTS to target your snmp machine
- [x] `set PASSWORD public`
- [x] `set PASS_FILE ''`
- [x] `run`
- [x] VERIFY public is successful
- [x] VERIFY the successful cred appears in `creds`