Switch provisioners to install Boost from GitHub releases on Windows (#972)

* switch provisioners to install boost from github releases

* minor fixes

* separate boosts by toolset versions

* minor fix

* switch provisioners for win16

* fix bugs

* fix validation

* minor fix

* fix validation

* fix validation

* fix validation

* minor fix

* change toolset version for win16

* minor fixes

* minor fix

* fix validation

* update links to toolset-json

* add arch to BoostInstallationDir

images/win/Windows2016-Azure.json

@@ -612,7 +612,8 @@
         {
             "type": "powershell",
             "environment_vars": [
-                "ROOT_FOLDER={{user `root_folder`}}"
+                "ROOT_FOLDER={{user `root_folder`}}",
+                "TOOLSET_JSON_PATH={{user `toolset_json_path`}}"
             ],
             "scripts":[
                 "{{ template_dir }}/scripts/Installers/Install-Boost.ps1"
@@ -782,15 +783,6 @@
                 "{{ template_dir }}/scripts/Installers/Validate-Go.ps1"
             ]
         },
-        {
-            "type": "powershell",
-            "environment_vars": [
-                "ROOT_FOLDER={{user `root_folder`}}"
-            ],
-            "scripts":[
-                "{{ template_dir }}/scripts/Installers/Validate-Boost.ps1"
-            ]
-        },
         {
             "type": "powershell",
             "scripts":[