Avoid redefining host to none in get_instance_nw_info(...)

The host parameter is required and a part of the method signature.
When it is redefined from kwargs.get('host') the supplied value
gets redefined to None. This removes the redefinition and keeps the
parameter value intact.

Fixes bug #1194178

Change-Id: I779cbce909935a4b794c16cefb0128f16f8563fb
(cherry picked from commit b0a2a2d29f24cea15d866f352d4beae5da965605)
Merge "Avoid redefining host to none in get_instance_nw_info(...)" into stable/grizzly