task/ssh_keys: fix invalid escape sequence '\w'

task/ssh_keys: fix invalid escape sequence '\w'

Fix syntax warning message:

    <unknown>:68: SyntaxWarning: invalid escape sequence '\w'
    teuthology/task/ssh_keys.py:57: SyntaxWarning: invalid escape sequence '\w'
      match = re.match('[\w-]+ {key} \S+@\S+'.format(key=re.escape(ssh_key)), line_to_test)
    teuthology/task/ssh_keys.py:68: SyntaxWarning: invalid escape sequence '\w'
      match = re.match('[\w-]+ \S+ {username}@\S+'.format(username=username), line_to_test)

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>