Trim whitespace in CLICKHOUSE_SKIP_TABLES var

[zqin]

If there is whitespace between table names in the environment variable CLICKHOUSE_SKIP_TABLES, the whitespace is contained in the table name. It looks like table names are split by delimiter , only. Please trim whitespace from the table name in this case.

E.g.,

$ export CLICKHOUSE_SKIP_TABLES="db1.table1, db2.table2"

# db2.table2 is not skipped:
$ ./clickhouse-backup -c ./config.yml tables
db2.table2
....