lib/oe/package_manager.py (rpm): Allow use of non-signed packages

lib/oe/package_manager.py (rpm): Allow use of non-signed packages

When we wish to use the package feed for local development, it does
not uses GPG signed feeds by default but dnf uses package signature
check. We need to configure the GPG signature check out so it works
out of box.

With this patch, installing non-signed packages works:

$: dnf install <package>

(From OE-Core rev: bceafe23cbe12a1ab0628b70865cb8867c7d7ee1)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>