Replace direct usage of SCYLLA environment variable with the build_mode
pytest fixture and path_to helper function. This makes tests more
flexible and consistent with the test framework. Also this allows to use
tests with xdist, where environment variable can be left in the master
process and will not be set in the workers
Add using the fixture to get the scylla binary from the suite, this will
align with getting relocatable Scylla exe.
The new option deprecates the existing `enable_tablets` option.
It will be extended in the next patch with a 3rd value: "enforced"
while will enable tablets by default for new keyspace but
without the posibility to opt out using the `tablets = {'enabled':
false}` keyspace schema option.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
