Add pagination to snapshots

Add pagination to snapshots

Currently snapshot list does not provide pagination like volume does.

This patch adds pagination for snapshots in the same way that volume
does, using marker, limit, sort_keys and sort_dirs.

DocImpact
APIImpact: Use marker, limit, sort_keys and sort_dirs
Implements: snapshots-and-bakcup-support-pagination-query
Implements: blueprint extend-limit-implementations
Change-Id: I21edac6d29abf7fc2f446111a7d6888db9ab6eba