Test: cts-lab: Remove RemotePrimitives class

Simplify slightly by dropping the RemotePrimitives class and adding its
only two class variables to the RemoteFactory class. Accordingly, split
the rsh instance variable of RemoteExec into command and cp_command
since the RemotePrimitives wrapper class is gone.

It looks like nothing used the RemoteExec.rsh member directly, so no
modifications are needed outside remote.py.

Closes T499

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>