NetworkPkg: Read HttpTlsCipherList variable and configure it for HTTPS session.

NetworkPkg: Read HttpTlsCipherList variable and configure it for HTTPS session.

v2:
* Refine the error handling returned from GetVariable.

This patch is to read the HttpTlsCipherList variable and configure it for the
later HTTPS session.

If the variable is not set by any platform, EFI_NOT_FOUND will be returned
from GetVariable service. In such a case, the default CipherList created in
TlsDxe driver will be used.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Cc: Zimmer Vincent <vincent.zimmer@intel.com>
Cc: Yao Jiewen <jiewen.yao@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>