Add disk_cachemodes and instances_path support

Add support for setting disk_cachemodes and instances_path configuration
flags.  Add initialization of instances_path directory with proper owner
and permission settings.

Partially implements: blueprint gpfs-storage-support
Change-Id: I07e0d648530461bd8e0e47f6427b10106bae3f1d
Merge "Add disk_cachemodes and instances_path support"