This allows hostapd to set a different management group cipher than the
previously hardcoded default BIP (AES-128-CMAC). The new configuration
file parameter group_mgmt_cipher can be set to BIP-GMAC-128,
BIP-GMAC-256, or BIP-CMAC-256 to select one of the ciphers defined in
IEEE Std 802.11ac-2013.
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>