Missed renaming for Sahara system tests

We can still see the word "savanna" in the names of variables, methods
and classes in system tests for Sahara. This fact causes errors when
running OSTF tests because all variables, methods and classes have been
renamed in OSTF tests for Sahara.

In addition, variable SERVTEST_SAVANNA_SERVER_URL has been removed
because there is no code fragment where this variable is used.

Closes-Bug: #1353433

Change-Id: Ia414f9f5bbb0a5efbab537f7f2a1b381dc7387ca
Merge "Missed renaming for Sahara system tests"