If user has specified the desired alter algorithm by using session
variable alter_algorithm, and not specifying the algorithm in the
ALTER SQL statement, then the ALTER will be processed in sending node
according the algorithm chosen by the alter_algorithm variable.
But in receiving nodes, appliers cannot see this session variable,
and will use the default algorithm when executing the ALTER.

The fix in this commit, will check if user has set alter_algorithm
variable and has not specified the algorithm clause in the ALTER
statement, and in such case will rewrite the original ALTER statement
appended by the algorithm clause. This rewritten ALTER statement will
be used in replication.

The commit has also new mtr test: galera.galera_alter_table_algorithm
which verifies that ALTER query rewrite happens when needed
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>