Refactor performance suites to use keyword timeouts

This removes the ugly hack with ${XXX_TIMEOUT_FOR_TEST_CASE}
global variables and test timeouts for a more elegant
solution that uses a keyword timeout instead. Requires Robot
Framework 3.0.

Change-Id: If5876cce5edccfc693a06fff5a9c8c892b2da0f2
Signed-off-by: Jozef Behran <jbehran@cisco.com>