Replace hard-coded algorithms with variables

[seancale]

Hi, I noticed that there is no current way to override the default algorithms with a variable. In the config template, they are hard coded to SHA and AES.

https://github.com/sbaerlocher/ansible.snmp/blob/d202875d5c7aaca2e2979065c4c4b6876766ad42/templates/snmp/snmpd.conf.j2#L3

These values should be replaced with variables in order to have the ability to substitute them with more or less secure options depending on needs.