Fix check instance host for instance action

When instance has no host, actions such as get_console_output,
start_stop_instance cause HTTP 500 response. Here change to
HTTPConflict when action called before host set.

Fix LP# 1116012

Change-Id: I6153a03f449d9fad8d0d8fb7295bdea4d2b2c2b1
Merge "Fix check instance host for instance action"