Feature/msp 10230/snmp login

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

include enumerable?  You implemented each so may as well.