Adds option to overwrite standard Post Deployment Actions Directory

Currently Kudu uses a standard and fixed location for Post Deployment Actions, which is the DeploymentToolsPath + "PostDeploymentActions". When running Kudu in private mode (not on WAWS) we were missing the ability to override the standard location, so we could add scripts to a central location in order to ensure that these scripts are run for all sites. This pull-request adds a non-intrusive option to set the path to a central Post Deployment Actions directory as an AppSetting in web.config.

What is does is to add a PostDeploymentActionsDirectory constant in the SettingsKeys for valid AppSettings, which is used to lookup the setting in web.config. An extension method called GetPostDeploymentActionsDir is added, which is called from the ExternalCommandFactory. The standard Post Deployment Actions Directory path is sent to the extension method, so if no AppSetting is explicitly set it will default to the regular path.
I hope you will consider to add this PR as it is "just" adding an optional setting ;-)