Prefix userid's with snap_ when used in a snap.

[micahl]

This change is to unblock potential development of a snap package for IoT Edge. A pre-requisite is to detect when running in snap confinement (e.g. looking at env vars injected by snapd) and then expect the OS userid's to be prefixed with snap_

Tested by installing a build of IoT Edge, creating the snap_* users, reassigning ownership of the various files/directories to the snap_ users, copying over the built 'iotedge' binary with this change, and using it to perform actions such as:

• sudo SNAP=1 ./iotedge config mp -c ...
• sudo SNAP=1 ./iotedge config apply

And then I manually inspected the generated config files and the user ID's registered. For example, here's the output from one:

micahl@fooD:~$ sudo cat /etc/aziot/identityd/config.d/00-super.toml
# This file is auto-generated by `iotedge config apply`
# Do not edit it manually; any edits will be lost when the command is run again.

hostname = "fooD"
homedir = "/var/lib/aziot/identityd"

[provisioning]
source = "manual"
iothub_hostname = "micahl-test-hub.azure-devices.net"
device_id = "doctest"

[provisioning.authentication]
method = "sas"
device_id_pk = "device-id"

[[principal]]
uid = 988
name = "aziot-edge"

micahl@fooD:~$ id 988
uid=988(snap_iotedge) gid=994(snap_iotedge) groups=994(snap_iotedge),101(systemd-journal),121(docker),998(snap_aziotks),996(snap_aziotcs),995(snap_aziotid)