sanitycheck: allow N platforms per arch to be run

It is an unfortunate fact of life that our CI machines are often
severly underpowered virtual machines that take quite some time
to run sanitycheck --all. The only other option was to just use
the default test case filtering semantics which chose one platform
per arch to test.

Now it is possible to specify N platforms per arch with the new
--platform-limit option, allowing greater flexibility on how many
tests will be run.

The counter-intuitive use of '--platform all' or '--platform default'
is no longer supported as these use-cases are covered by other
command line options.

Change-Id: I45b0050df52e1a22e75534a3876d89e3c77698c9
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>