Adds the ability to sort images with multiple keys

Extend rest api v2 with multiple sort keys support.
Example:
/v2/images/detail?sort_key=name&sort_key=size
Changed database api which now takes sort_key param as a list instead of string
python-glanceclient support will be added in separate commit

Change-Id: Ib7a6aeb2df3bc5d23fe8e070290b5bfcab00c0f5
DocImpact
Partial-Bug: 1221274
Merge "Adds the ability to sort images with multiple keys"