Fix target host variable from being overwritten

The target host parameter of the compute api evacuate method
is mistakenly overwritten before being used. The result is
that the instance rebuild operation is tried against the
original failed host and will always fail.

Rename locally used variable to a different name and modify
relevant test to prohibit this from happening again.

Resolves bug 1133204.

Change-Id: I517b2dc19d9561723391150105b4943b56dba637