[SNMP] added rfc 2578 compliant instance support

It's done :) ... onward to the very first pull request in my life ( disregarding the request I canceled yesterday :P ).


#### Instance support

Instance support is now working in compliance with rfc 2578 -
to achieve this, an additional XML "instance" property was added to the template -
if property does not exist, instance "0" is being assumed.

Tested with the following symbols:

```xml
 <mib name="IF-MIB">

                <symbol name="ifNumber">
                    <value>2</value>
                </symbol>

                <symbol name="ifIndex" instance="1">
                    <value>1</value>
                </symbol>

                <symbol name="ifIndex" instance="2">
                    <value>2</value>
                </symbol>

                <symbol name="ifDescr" instance="1">
                    <value>Ethernet Port 0, link, 100 Mbit, full duplex, autonegotiation</value>
                </symbol>

                <symbol name="ifDescr" instance="2">
                    <value>Ethernet Port 1, link, 100 Mbit, full duplex, autonegotiation</value>
                </symbol>

</mib>

<mib name="IP-MIB">
    <symbol name="ipAdEntIfIndex" instance="192.168.1.200">
        <value>1</value>
    </symbol>
</mib>

```


#### Error handling for unknown symbols

conpot now informs the user of symbols it could not find within a
MIB and skips the respective XML stanza instead of crashing.