lib/oe/lsb: make the release dict keys consistent regardless of source

lib/oe/lsb: make the release dict keys consistent regardless of source

Rather than have the distro_identifier method look for different keys in
the dict depending on the source ensure that each function for retrieving
release data uses the same key names in the returned dict.

(From OE-Core rev: 2ddd6ddaf0c5ba14ae83347eba877ac9ef179c76)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>