Raise appropriate exception if socket error occurs

Added a custom exception RemoteServiceUnavailable in
glance_store.exceptions and raised it from "get" method of http store
in case of socket.error exception is found.

Moved mocking httplib request and response logic from setup method
to a private method and called it only from the unit test cases where
it is required.

Partial-Bug: #1379798
Change-Id: I065b9a3e8e674ea74ff8563aad99d7d022417caa
Merge "Raise appropriate exception if socket error occurs"