Fix rebuild server tests for wrong input

Nova rebuild server API takes 'metadata' as one of the input in
request body. But test_rebuild_non_existent_server pass
wrong input 'meta' in rebuild server call.

This patch removes optional arg of rebuild call from this test.

This tests does not fail because Nova V2 does not have strong input validation.
It is failed in Nova V2.1 experimental job on-
Ib13b02cefb9617e8af3b62b81d2f7eb7cd88eab1

Change-Id: Id276b94ae757801babc5e026ee5e38179d9e09e8
Merge "Fix rebuild server tests for wrong input"