Store KeyProvider in ClientContext to avoid leaking key provider threads when using FileContext

Currently, the FileContext class used by clients such as (for eg. YARNRunner) creates a new AbstractFilesystem object on initialization.. which creates a new DFSClient object.. which in turn creates a KeyProvider object.. If Encryption is turned on, and https is turned on, the keyprovider implementation (the KMSClientProvider) will create a ReloadingX509TrustManager thread per instance... which are never killed and can lead to a thread leak