Use current time as default start validity time for certs and permissions

[mikaelarguedas]

The validity time used is "a day before" as of #209
This is an arbitrary workaround introduced because RTI Connext (5.3.1) thinks the time extracted from the permission is "in the future".
The assumption is that connext consider the extracted time as a time in the local timezone and not UTC, while the DDS Security specification states:

<!-- Format is CCYY-MM-DDThh:mm:ss[Z|(+|-)hh:mm] The time zone may
be specified as Z (UTC) or (+|-)hh:mm. Time zones that aren't
specified are considered UTC.
-->
<not_before>2013-10-26T00:00:00</not_before>
<not_after>2018-10-26T22:45:30</not_after>

Implementation considerations

We attempted to make the timezone explicit but it didn't fix the issue #210