Fix sh parse error when [sst] config value has spaces

Example config:

[sst]
sst-syslog=-1
progress=/tmp/mysql-console/fifo
compressor="pigz --fast --processes 4"
decompressor="pigz --decompress"
Errors reported during SST:

/usr//bin/wsrep_sst_common: line 261: [: too many arguments
/usr//bin/wsrep_sst_common: line 261: [: pigz: binary operator expected