swift: change eqiad to the new partition layout

Change create_filesystem/mount_filesystem targets to swap sdm with sda
and sdn with sdb in accordance with the new partitioning layout.
See Id0df94 for the rationale.

Change-Id: I62c16fc7a7f2356d2afb722d4aa66d6a4e5bc166
Merge "swift: change eqiad to the new partition layout" into production