Bug 37495: Add ability to use metadata to filter plugins to run for plugins_nightly.pl

Bug 37495: Add ability to use metadata to filter plugins to run for plugins_nightly.pl

It would be nice to be able to filter the plugins run by plugins
nightly for development and testing, and for added flexibility
as to when to run cronjob_nightly for different plugins if need be.

Test Plan:
1) Apply this patch
2) Check the metadata for an installed plugin, copy the name or another
   metadata value.
3) Run plugins_nightly.pl with a filter that does not match
   e.g. plugins_nightly.pl -m name="No plugin has this name"
4) Note no plugins cronjob methods are run
5) Run plugins_nightly.pl with a filter that *does* match
   e.g. plugins_nightly.pl -m name="Example Kitchen-Sink Plugin"
6) Not only the matching plugin runs!
7) Run plugins_nightly with no filter
   e.g. plugins_nightly.pl
8) Note all plugins with nightly cronjobs are run!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>