A simple convenience configfile can provide a mapping between VPP
interface names, Linux Control Plane interface names, and descriptions.
An example:
```
interfaces:
"TenGigabitEthernet6/0/0":
description: "Infra: xsw0.chrma0:2"
lcp: "xe1-0"
"TenGigabitEthernet6/0/0.3102":
description: "Infra: QinQ to Solnet for Daedalean"
lcp: "xe1-0.3102"
"TenGigabitEthernet6/0/0.310211":
description: "Cust: Daedalean IP Transit"
lcp: "xe1-0.3102.11"
```
This configuration file is completely optional. If the `-c` flag is
empty, or it's set but the file does not exist, the Agent will simply
enumerate all interfaces, and set the `ifAlias` OID to the same value
as the `ifName`. However, if the config file is read, it will change
the behavior as follows:
* Any `tapNN` interface names from VPP will be matched to their PHY by
looking up their Linux Control Plane interface. The `ifName` field
will be rewritten to the _LIP_ `host-if`. For example, `tap3` above
will become `xe1-0` while `tap3.310211` will become `xe1-0.3102.11`.
* The `ifAlias` OID for a PHY will be set to the `description` field.
* The `ifAlias` OID for a TAP will be set to the string `LCP: `
followed by its PHY `ifName`. For example, `xe1-0.3102.11` will
become `LCP TenGigabitEthernet6/0/0.310211 (tap9)`
Now that we're explicitly connecting via TCP to localhost:705 (which
can be overriden by the -a flag), we no longer need to run as root.
Therefore, update vpp-snmp-agent.service to run as user Debian-snmp
group vpp, so that /run/vpp/{api,stats}.sock are writable.
Be explicit on the commandline arguments in the service definition.
