Updated pip module internal check for log.

[abednarik]

When using pip you can append --log to store pip logs. In this case i made a change to make sure
argument to --log is not a directory, other way pip will throw and error. Also we need to make sure that
we have write permissions to that file.
Finally i made a small update on this file regarding indentation following pep8 standards.

Related to issue #29470

In this case, since is a feature hope is ok to make this PR against develop.

[abednarik]

Hmm...a test related to this commit is failing. It's just fin that if a directory is used us an argument, this will fail, since pip also throw an error. I don't get it why tests fail like this

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/testing/tests/unit/modules/pip_test.py", line 326, in test_install_log_argument_in_resulting_command
    pip.install(pkg, log=log_path)
  File "/testing/salt/modules/pip.py", line 531, in install
    '\'{0}\' is a directory. Use --log path_to_file'.format(log))
IOError: '/tmp/pip-install.log' is a directory. Use --log path_to_file

Thi sis the test

    @patch('os.path')
    def test_install_log_argument_in_resulting_command(self, mock_path):
        pkg = 'pep8'
        log_path = '/tmp/pip-install.log'
        mock = MagicMock(return_value={'retcode': 0, 'stdout': ''})
        with patch.dict(pip.__salt__, {'cmd.run_all': mock}):
            pip.install(pkg, log=log_path)
            mock.assert_called_once_with(
                ['pip', 'install', '--log', log_path, pkg],
                saltenv='base',
                runas=None,
                cwd=None,
                use_vt=False,
                python_shell=False,
            )

Any advice, help is much appreciated.

[cachedout]

@jfindlay Could you help out here please?