Handled corner case scenarios & enabled functional tests

Enabled execution of functional tests by sourcing devstack
credentials and executing 'tox -eintegration'.
Handled following corner case scenarios, where stack status was
incorrect:
1. stack update with same (create time) template: stack status:
UPDATE_IN_PROGRESS
2. stack update with just one resource addition: stack status:
UPDATE_GC_IN_PROGRESS
3. create with no resource: stack status CREATE_IN_PROGRESS
4. deleting stack with no resource: DELETE_IN_PROGRESS

Since (most) resources are removed, tweaked testr conf to consider
only functional tests from the heat_integrationtests dir.
Similarly tweaked setup.cfg to ignore the missing resource errors.