Removed use_first_core from the conf.yaml in favor of determining this
within the framework. use_first_core continue to serve a purpose in that
it is only enabled when core 0 is explicitly provided in the
configuration. Any other configuration, including "" or "any," will
omit core 0.

Documentation reworks are included to reflect the changes made.

Bugzilla ID: 1360
Signed-off-by: Nicholas Pratte <npratte@iol.unh.edu>