oeqa/target/ssh: add ignore_status argument to run()

The underlying _run() method has an ignore_status argument and can
either return the exit code or raise an exception if it is non-zero.

Add the same argument to the run() method so that test cases can change
this value. It currently defaults to True as that is the existing
behaviour, but a follow-up patch will change this to False as test cases
should fail on errors unless told otherwise.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>