cli: Add provision for default benchmark properties

Some some new properties to a benchmark object within the plan:

  - warmups: Number of iterations of the benchmark to run at the start
    of a session to warm the system up. Results from warmup iterations
    are discarded. Must be 0 or more. Defaults to 0.
  - repeats: Number of iterations of the benchmark to run for each
    session after the system is warm. Results from each of these
    iterations are kept. Must be 1 or more. Defaults to 3.
  - sessions: Number of reboot sessions to run the benchmark under. Must
    be 1 or more. Defaults to 1.
  - timeout: Duration to wait for a benchmark iteration to complete. If
    it hasn't completed within the timeout, forcibly kill it and mark
    the iteration as having an error. Defaults to 1 hour.

These properties can all be set in a new global "defaults" section
within the plan, and any values provided here will be automatically
applied to all benchmarks that don't explcitly override the properties
with custom values.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>