Use legacy btrfs API in RHEL6 tech preview

Anaconda requires special commands for btrfs in recent versions,
but the RHEL6 tech preview treats it like any other file system.

Bug: 1002261
Change-Id: Ifec4dc1bc94b728f98f2d3335896880184e0019a
Merge "Use legacy btrfs API in RHEL6 tech preview" into develop