Solve issue at runbackup umounting previous backups

[didacog]

Disaster Recovery Linux Manager (DRLM) Pull Request Template

Please fill in the following items before submitting a new Pull Request:

PR details:

• PR type (Bug Fixing / New Feature / Enhancement / Other?): Enhancement
• Reference to related issue (issue link):
• Impact (Low / High / Critical / Urgent): low
• Did you test this PR? yes
• Brief description of the changes in this PR: force umount at runbackup time when disabling previous backups