Galera mtr tests currently use wsrep_provider as /usr/lib/galera/libgalera_smm.so.
However, on 64-bit platforms galera library gets installed under /usr/lib64/galera.
The idea is to provide an environment variable WSREP_PROVIDER that should
hold the correct location for the galera library.
This environment variable can also be used to run tests with galera library located
at non-standard locations by setting it accordingly.