ktest: Add OVERRIDE keyword to DEFAULTS section

The OVERRIDE keyword will allow options defined in the given
DEFAULTS section to override options defined in previous DEFAULT
sections.

Normally, options will error if they were previous defined.
The OVERRIDE keyword allows options that have been previously
defined to be changed in the given section.

Note, the same option can not be defined in the same DEFAULT section
even if that section is marked as OVERRIDE.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>