Porting v3 evacuate testcases to v2

Ported test cases:
test_evacuate_instance_with_underscore_in_hostname()
test_evacuate_instance_with_empty_host()

and some other code refactor in evacuate v2 testcases.

Partially implements blueprint v2-on-v3-api
Change-Id: Ibe13526ef674869593a14c719062652f59ce2345
Merge "Porting v3 evacuate testcases to v2"