[cassini-bsp, cassini-tests] Fix parsec RSA key generation failure

The RSA key generation operation is very slow and this causes the
parsec-tool to timeout when the cli tests are executed. The parsec cli
test script can use a preset PARSEC_TOOL variable so this patch sets
this variable with sufficient timeout to complete the tests.

Signed-off-by: Gowtham Suresh Kumar <gowtham.sureshkumar@arm.com>