backup_mysql.sh via 7zip

======== Required information ====
- iRedMail version: 0.9.0
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MysQL
- Web server (Apache or Nginx): Apache2
- Linux/BSD distribution name and version:  Ubuntu Server LTS
- Related log if you're reporting an issue:
====

If someone interested attacched a variation to the backup_mysql.sh script set to compress via 7zip
The only modification to do is change the four following lines:

080: export CMD_COMPRESS='7z -mx=9 -bd  -t7z a '
154:            ${CMD_COMPRESS} ${output_sql}.7z ${output_sql} >>${LOGFILE}
161:            if echo ${CMD_COMPRESS} | grep '^7z' >/dev/null; then
162:                compressed_file_name="${output_sql}.7z"

JC