Update log message for add_host_to_aggregate

When add host to aggregates with availability zone, if the host
is already in one availability zone, then the host cannot be added
to another availability zone.

When user do above operation, nova api will report error message,
but the error message does not include blank space between
<availability zone> and zone name, also the error message include
two periods.

The fix was adding blank space and removes one period for the message.

Change-Id: I7307288be194d3ce6de05b77433bbe2626ae46d9
Closes-bug: #1237303
Merge "Update log message for add_host_to_aggregate"