mender-artifact cp misbehaves when stdin and two arguments are passed

mender-artifact cp command is design to imitate the Linux standard tools with regards to how to treat arguments and stdin.

For the following command:

cat otherfile | mender-artifact cp myfile myartifact:/home/myfile

• Current behavior: takes otherfile from stdin and try to copy it into myfile, failing due to myfile not being a Mender Artifact nor a SD image.
• Expected behavior: ignores stdin, executes cp from myfile into myartifact.

Acceptance criteria:

• The cp command behaves like the Linux standard tools: first check the arguments, if you don't have enough then check stdin.
• Optionally: use "-" argument to specify the use of stdin in the cp command. Without this argument, stdin should always be ignored.

Edit:

Acceptance criteria:

• The cp command explicilty requires '-' in order to read from stdin. This is preferred over the regular gnu-tools functionality because it keeps black magic at bay. This means that two command line arguments are always required.