"Stop Tests" button works only once

Seems like "Stop tests" button behavior is not worked properly.
User can stop Sanity tests (or any other) but once. Next time "Stop tests" button will be disabled (9.0) or in progress (10.0 round twisted icon, see screenshots).

Steps to reproduce:
1. Deploy any environment
2. Open Health Check tab
3. Start Sanity tests
4. Click button "Stop tests"
5. Start Sanity tests again (or any other tests)
6. Click button "Stop tests"

Expected result:
"Stop tests" button available and will stop tests execution.

Actual result:
"Stop tests" button disabled. User can not stop tests execution more than one time.

Screenshots in attach. At screenshots steps 5-6 reproduced two times, for Sanity and Functional tests.

[root@nailgun ~]# shotgun2 short-report
cat /etc/fuel_build_id: 521
cat /etc/fuel_build_number: 521
cat /etc/fuel_release: 9.0
cat /etc/fuel_openstack_version: mitaka-9.0
rpm -qa | egrep 'fuel|astute|network-checker|nailgun|packetary|shotgun':
 fuel-release-9.0.0-1.mos6349.noarch
 fuel-misc-9.0.0-1.mos8460.noarch
 python-packetary-9.0.0-1.mos140.noarch
 fuel-bootstrap-cli-9.0.0-1.mos285.noarch
 fuel-migrate-9.0.0-1.mos8460.noarch
 rubygem-astute-9.0.0-1.mos750.noarch
 fuel-mirror-9.0.0-1.mos140.noarch
 shotgun-9.0.0-1.mos90.noarch
 fuel-openstack-metadata-9.0.0-1.mos8743.noarch
 fuel-notify-9.0.0-1.mos8460.noarch
 nailgun-mcagents-9.0.0-1.mos750.noarch
 python-fuelclient-9.0.0-1.mos325.noarch
 fuel-9.0.0-1.mos6349.noarch
 fuel-utils-9.0.0-1.mos8460.noarch
 fuel-setup-9.0.0-1.mos6349.noarch
 fuel-provisioning-scripts-9.0.0-1.mos8743.noarch
 fuel-library9.0-9.0.0-1.mos8460.noarch
 network-checker-9.0.0-1.mos74.x86_64
 fuel-agent-9.0.0-1.mos285.noarch
 fuel-ui-9.0.0-1.mos2717.noarch
 fuel-ostf-9.0.0-1.mos936.noarch
 fuelmenu-9.0.0-1.mos274.noarch
 fuel-nailgun-9.0.0-1.mos8743.noarch

[root@nailgun ~]# shotgun2 short-report
cat /etc/fuel_build_id: 393
cat /etc/fuel_build_number: 393
cat /etc/fuel_release: 10.0
cat /etc/fuel_openstack_version: newton-10.0
rpm -qa | egrep 'fuel|astute|network-checker|nailgun|packetary|shotgun':
 nailgun-mcagents-10.0.0-1.mos760.noarch
 shotgun-10.0.0-1.mos92.noarch
 fuel-agent-10.0.0-1.mos302.noarch
 fuel-ostf-10.0.0-1.mos945.noarch
 fuel-mirror-10.0.0-1.mos145.noarch
 fuel-release-10.0.0-1.mos6362.noarch
 python-fuelclient-10.0.0-1.mos346.noarch
 network-checker-10.0.0-1.mos77.x86_64
 fuel-10.0.0-1.mos6362.noarch
 fuel-openstack-metadata-10.0.0-1.mos8835.noarch
 fuel-nailgun-10.0.0-1.mos8835.noarch
 fuel-migrate-10.0.0-1.mos8601.noarch
 fuel-setup-10.0.0-1.mos6362.noarch
 rubygem-astute-10.0.0-1.mos760.noarch
 fuel-misc-10.0.0-1.mos8601.noarch
 fuel-library10.0-10.0.0-1.mos8601.noarch
 fuelmenu-10.0.0-1.mos289.noarch
 fuel-utils-10.0.0-1.mos8601.noarch
 fuel-notify-10.0.0-1.mos8601.noarch
 python-packetary-10.0.0-1.mos145.noarch
 fuel-ui-10.0.0-1.mos2807.noarch
 fuel-bootstrap-cli-10.0.0-1.mos302.noarch