Set minimal tempest version to 27.0.0

As tempest.scenario.manager was announced stable interface in Tempest
27.0.0[1] it can be now reused in plugins.

Some methods replaced in scenario-manager-cleanup effort won't work
with tempest version lower than 27.0.0 therefore minimal tempest
version must be updated in tempest plugins

Etherpad concerning this effort:
https://etherpad.opendev.org/p/tempest-scenario-manager-cleanup

[1] https://docs.openstack.org/releasenotes/tempest/v27.0.0.html#release-notes-27-0-0

Change-Id: I422435abd4fb34fbba187deb0d96ef314767ff19
diff --git a/requirements.txt b/requirements.txt
index 67ae150..f2e2e43 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,7 +4,7 @@
 
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 
-tempest>=17.1.0 # Apache-2.0
+tempest>=27.0.0 # Apache-2.0
 cryptography>=2.1 # BSD/Apache-2.0
 
 oslo.config>=5.2.0 # Apache-2.0