Improving help text for Swift store util opts.
Improving help text for Swift store util opts.

Adding improved help text for:
swift_store_ssl_compression
swift_store_retry_get_count
swift_store_expire_soon_interval
swift_store_use_trusts

Change-Id: I1bcd8415e7b93b7693cc950d5d6531c770147206
Partial-Bug: #1570946

openstack/glance_store,openstack/glance_store