mandos-ctl: Refactor; move parsing of intervals into argument parsing
* mandos-ctl (MillisecondsValueArgumentMixIn.value_to_set): Assume that an incoming value is datetime.timedelta(), not a string. (add_command_line_options): Add "type=string_to_delta" to --timeout, --extended-timeout, --interval, --approval-delay, and --approval-duration. (TestSetTimeoutCmd, TestSetExtendedTimeoutCmd, TestSetIntervalCmd, TestSetApprovalDelayCmd, TestSetApprovalDurationCmd): Change values_to_set to be datetime.timedelta() values, and change to more appropriate values to test. Also adjust values_to_get accordingly.