Migrate volume should check para "host" in request

The server doesn't check whether the parameter "host" is in request
body. So the 500 error has been thrown.

We should catch the KeyError and transfer the KeyError to
400 (HTTPBadRequest) instead of 500.

Closes-Bug: #1253904
Change-Id: I3fb07113816a87f284b47e32bacd57f78a32676c
Fix docstring for _migrate_volume_completion

"Migrate a volume to the specified host." as the description of
_migrate_volume_completion func is incorrect.

Closes-Bug: #1253910
Change-Id: Ica944d9a7fa962ff56a9f0107ac1519623d2e551
Initialize and terminate connection raise 500 err

Initialize and terminate connection should check whether "connector" is
in request body. It throws 500 error if "connector" is not present. We
should catch the KeyError and transfer it to 400 (HTTPBadRequest)
instead of.

Closes-Bug: #1253944
Change-Id: If38419592701c8a14df52f94fd46ed0fc7a17e04
Merge "Migrate volume should check para "host" in request"
Merge "Initialize and terminate connection raise 500 err"
Merge "Fix docstring for _migrate_volume_completion"