Added CMPIContext to CMPIProxyProvider

When CMPIProxyProvider loads a python module and calls its functions (e.g.
get_providers), it should provide full 'env' parameter to these functions.

My provider is multi-threaded and creates some threads as early as in
get_providers function. For this I need env.get_cimom_handle() to work there so
I can call PrepareAttachThread / AttachThread. Without this patch I get
SIGSEGV, since the returned CIMOM handle is None.