Add with_retcode option to ShellCase.run_call(). Also adds a test case for https://github.com/saltstack/salt/issues/6973 - expect a non-zero exit code when running "salt-call --retcode-passthrough state.highstate"
Add with_retcode options for ShellCase functions
Merge pull request #7224 from dlanderson/shell_test_retcodes

Add with_retcode option to ShellCase.run_call().