mgr/dashboard: encode non-ascii string before passing it to exec_cmd()

mgr/dashboard: encode non-ascii string before passing it to exec_cmd()

because on Python3, tempfile.TemporaryFile() is opened in binary mode by
default, we need to encode non-ascii string before write to it.
otherwise, we have following failure:

self = <dashboard.tests.test_access_control.AccessControlTest testMethod=test_unicode_password>

    def test_unicode_password(self):
        self.test_create_user()
        password = '\u7ae0\u9c7c\u4e0d\u662f\u5bc6\u7801'
        with tempfile.TemporaryFile(mode='w+') as pwd_file:
>           pwd_file.write(password)
E           UnicodeEncodeError: 'latin-1' codec can't encode characters in position 0-5: ordinal not in range(256)

tests/test_access_control.py:576: UnicodeEncodeError

this change is not cherry-picked from master, as master has dropped
python2 support.

Signed-off-by: Kefu Chai <kchai@redhat.com>