pybind/mgr/restful: move global variable into context.py

pybind/mgr/restful: move global variable into context.py

otherwise we need to tackle with the circular import. it's not fatal, if
we can "import foo" in py2 and "from . import foo" in py3, respectively.
but we cannot unify them using "from . import foo", because the circular
reference issue. and conditionalize the import statement is ugly. so, i
think we'd better break the loop. so i am moving the "instance" variable
into its own file.

Signed-off-by: Kefu Chai <kchai@redhat.com>