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.
nose is no longer maintained, thus we have to move to another
testing system. This commit updates the tox setup and all tests
to use pytest instead of nose.
