mender-artifact snapshot requires sudo on the device

When creating an Artifact using mender snapshot command fails with the following error if sudo does not exist on the device.

sh: sudo: command not found

This would be typical for Yocto built images, where you necessarily do not have sudo installed and the only user by default is root.

Acceptance criteria:

• Do not attempt to run sudo if snapshot is performed as root.
• Update acceptance test to assess sudo is not invoked on root user.