Fix device mount/umount methods in generic driver

Generic driver has methods for mounting block devices and unmounting.
They have logic, that depends on exception messages.
This change removes dependency on messages of exceptions in mentioned
methods to avoid distro-incompatibility.

Changes:
- added new method '_is_device_mounted'
- rewritten method '_mount_device'
- rewritten method '_unmount_device'
- rewritten unittests
- added locks to mount/unmount methods by share-id

Change-Id: I21500c83c00640d13f40ebb992d59d311fd3ef1e
Closes-Bug: #1359196
Merge "Fix device mount/umount methods in generic driver"