Command.run and Command.call now make use of Path.which in order
to test if the command is available or not. Also Path.which has
been extended in order to support custom environments and to be
able to test file permissions (read. write, execute)
This commit includes a validation in Command.run and Command.call
in order to verify the existance of the command before running it.
It case it is not found in the specified environment it raises a
KiwiCommandNotFound Exception.
In python2 bytes is string which is different from the bytes
type in python3. The bytes type from the builtins generalizes
this type to be bytes always. However the redefinition of the
bytes type is marked as Smell in landscape. Thus the code
should at least inform why this is done
Also log the stdout data from a failed command. So far we
expected error data on stderr but there are also commands
which print error messages on stdout. It should us at least
worth a debug message with this information
