Omitting timestamp from msfconsole search output

SeeRM #8795

The disclosure date field in the results from the search command
where returning with a timestamp that was almost always 00:00:00 UTC. I added a bit of date time formatting to only
include the year (4 digit), month (2 digit), and day (2 digit)
in the following format: Y-m-d.  This date time formatting
applies to both searches conducted through the database instance
as well as searches performed without a database (slow search).
Fixing nil bug and making format constant

The date format has been moved into a constant variable.
Certain modules do not have a disclosure_date.  For example,
‘checkvm’.  This necessitated checking disclosure_date for nil
before attempting a format conversion.  Also, there was an additional
location in core.rb that needed the formatting / nil check added.  Specs
were also updated appropriately.
Land #3324, msfconsole search timestamp fixes