Create tests for multiple server creation

This tests nova multiple servers creation using min_count and max_count
instance counters and return_reservation_id parameter.

Cause of the new tests this also add min_count and max_count to xml server
client and return_reservation_id to both clients.

Depends on changes:
    Ifaa9910e13614554d1769e71e8eba9587ec5a13b
    I3ca0129086ff1df3a8faa2b3b8bd959c9b74103c
Both on Nova.
Partially implements bp missing-tempest-tests

Change-Id: I4195b26d88632c2e679b7a6e5bd8cf9443d42553
3 files changed