Remove create_timestamp & get_timestamp methods

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

Replaced methods:
	* create_timestamp
	* get_timestamp

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: Id6ba585c58c0e9b6564d85137fd00ae2a8954c06
1 file changed