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: I0a9fa64a61e90b9a6ac4786222be6732da247594
diff --git a/requirements.txt b/requirements.txt
index e012abf..cde72ba 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -8,4 +8,4 @@
 oslo.serialization!=2.19.1,>=2.18.0 # Apache-2.0
 oslo.utils>=3.33.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
-tempest>=17.1.0 # Apache-2.0
+tempest>=27.0.0 # Apache-2.0