Move the test_utils module to scenario test dir

The test_utils module in tempest.common.utils is only useful for
scenario tests. So it's a little confusing to have it in the common
directory. This commit corrects this by moving the file into the
scenario directory. The service tag requirement for scenario tests
hacking check is updated to only check scenario test files, because
a method in the new utils module was being picked up incorrectly.

Change-Id: I1d70745a5ee07923e680328e5b8a29bfb82d8099
Add load_test mechanism for InputScenarioUtils

Similar to what was done for the negative test framework this commit
adds a method to handle using CONF values with testscenarios.
Previously the attr's were set in the test class definition which
would force a getattr at import time. This will cause issues if the
config file is not present. By adding a method which won't be executed
at import but rather when the tests are run we can avoid this.

Change-Id: Ib635f8b895f6642f78a3af644170700f72259d46
Merge "Move the test_utils module to scenario test dir"
Merge "Add load_test mechanism for InputScenarioUtils"