oeqa.utils.commands: limit runCmd exception output

oeqa.utils.commands: limit runCmd exception output

Make it possible to limit the length of output lines shown in runCmd
exceptions. E.g when running bitbake we easily get thousands of lines of
log output, where only the last few (tens) are interesting or relevant
when an error occurs.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>