Add ability to specify snmp config for network nodes

[jonstout]

Add ability to specify snmp community string for network nodes. When specified, this will allow for network statistics to be gathered and graphed on the frontend.

[jonstout]

SNMP options are as follows:

snmpv3:
    security_level: noAuthNoPriv|authNoPriv|authPriv
    auth_protocol: MD5|SHA
    auth_password: ''
    priv_protocol: DES|AES
    priv_password: ''
snmpv2:
    community: ''

[jonstout]

We can overload the passwords.json file to include the following special keys containing creds. They can then be assigned to a node by id via the frontend.

{
    "snmpv3": [
        {
            "id": "mlxe16-1.example.com",
            "security_level": "noAuthNoPriv|authNoPriv|authPriv",
            "auth_protocol": "MD5|SHA",
            "auth_password": "",
            "priv_protocol": "DES|AES",
            "priv_password": ""
        }
    ],
    "snmpv2": [
        {
            "id": "mlxe16-1.example.com",
            "community": ""
        }
    ],
    "ssh": [
        {
            "id": "mlxe16-1.example.com",
            "username": "",
            "password": ""
        }
    ]
}