Use existing arch/os information for searches and index operations

This code is a first step in making the current registry aware of
images for non-Intel architecture.  While os==linux today, if
we are adding architecture we might as well add both `arch` and
`os` for future use by Microsoft and/or others where os != linux.

Signed-off-by: Pradipta Kr. Banerjee <bpradip@in.ibm.com>
Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>