Allow group cipher selection to be overridden

Allow group cipher selection to be overridden

The new hostapd configuration parameter group_cipher can now be used to
override the automatic cipher selection based on enabled pairwise
ciphers. It should be noted that selecting an unexpected group cipher
can result in interoperability issues and this new capability is mainly
for testing purposes.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>