Allow - in Warden user name

- POSIX user name allows . and - in addition to character class \w
  http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_431
  http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_278
  https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html#sum

Change-Id: I5529ee0d89be0bcc3b92f955d09834e30cbcfea4

@@ -169,7 +169,7 @@ class Warden {
     synchronized String borrowCell(String userName, String sshKey, int minutes,
                                    String cellSpec) {
         checkNotNull(userName, USER_NOT_NULL);
-        checkArgument(userName.matches("[\\w]+"), "Invalid user name %s", userName);
+        checkArgument(userName.matches("[\\w.-]+"), "Invalid user name %s", userName);
         checkNotNull(sshKey, KEY_NOT_NULL);
         checkArgument(minutes < MAX_MINUTES, "Number of minutes must be less than %d", MAX_MINUTES);
         checkArgument(minutes >= 0, "Number of minutes must be non-negative");