Method to filter non-root block device mappings

Adding a generator that would provide a non-root block device
mappings, when it's optional variable exclude_root_mapping is
set to true. Otherwise, all mappings will be returned.

The method will be used to handle LXC volumes, as it's root FS
should be handled differently.

Change-Id: I879916021c3b61f19dd69ff11838dbbac19f72d1
Related-Bug: #1269990
Merge "Method to filter non-root block device mappings"