_error_in_logs function of driver.py shall check all nodes

1. Don't simply return, if one node is clean or not clean.
2. It's better to log all errors and return all errors.

Change-Id: Ib0e23bff013836347f90bed50dd1d3a9566e6bad
Close-Bug: #1229957
add server suspend/resume negative tests

- suspend_non_existent_server
- suspend_server_invalid_state
- resume_non_existent_server
- resume_server_invalid_state

Change-Id: I3aff0d6f017a9ed228e45307ecca53373e8daaf8
Handle test_list_servers_by_changes_since without tenant isolation

The test_list_servers_by_changes_since test fails when running with
allow_tenant_isolation=false because there is an arbitrary 2 minutes to
determine the timedelta for the changes-since in the query. That doesn't
work when the servers created in the test are all under the same
user/tenant because the tearDown doesn't block until the servers are
deleted.

This patch makes the timedelta code smarter in that it narrows the
changes-since down to when the test case started setting up the servers
for the test and when it does the query.

Closes-Bug: #1195309

Change-Id: Ie848637ea490da2151e5db8292726111d45a8c2d
TEMPEST_PY26_NOSE_COMPAT in py26 jobs

Use py26 skip exception compatibility code path in py26 jobs.

Change-Id: Id6c29f26f19f69abab558d5f94d8cbf6190ed43b
Merge "Handle test_list_servers_by_changes_since without tenant isolation"
Merge "add server suspend/resume negative tests"
Merge "_error_in_logs function of driver.py shall check all nodes"