NetworkPkg: Add PCD to enable the HTTP connections switch

v3:
* Correct the commits grammar

v2:
* Rename the PCD to PcdAllowHttpConnections.
* Refine the PCD descriptions.

If the value of PcdAllowHttpConnections is TRUE, HTTP connections are
allowed. Both the "https://" and "http://" URI schemes are permitted.
Otherwise, HTTP connections are denied. Only the "https://" URI scheme
is permitted.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>

NetworkPkg/HttpBootDxe/HttpBootDxe.inf

@@ -1,7 +1,7 @@
 ## @file
 #  This modules produce the Load File Protocol for UEFI HTTP boot.
 # 
-#  Copyright (c) 2015 - 2016, Intel Corporation. All rights reserved.<BR>
+#  Copyright (c) 2015 - 2017, Intel Corporation. All rights reserved.<BR>
 #  This program and the accompanying materials
 #  are licensed and made available under the terms and conditions of the BSD License
 #  which accompanies this distribution.  The full text of the license may be found at
@@ -94,5 +94,8 @@
   gEfiVirtualCdGuid            ## SOMETIMES_CONSUMES ## GUID
   gEfiVirtualDiskGuid          ## SOMETIMES_CONSUMES ## GUID
 
+[Pcd]
+  gEfiNetworkPkgTokenSpaceGuid.PcdAllowHttpConnections       ## CONSUMES  
+
 [UserExtensions.TianoCore."ExtraFiles"]
   HttpBootDxeExtra.uni