[SAP] modify select_datastore_by_name to use a cache

[hemna]

Optimize the select_datastore_by_name method to significantly reduce vCenter API load during high-volume operations like boot-from-volume.

Problem:
The original implementation called _get_datastores() which fetches ALL datastores with their 'host' and 'summary' properties. With ~40 datastores and ~50 hosts each, this transferred ~500KB-2MB of data per call. Under high load (e.g., 96 volume creates in 5 minutes), this caused vCenter connection pool exhaustion and cascading timeouts averaging 530 seconds per _select_ds_for_volume call.

Solution:

1. Add _get_datastore_by_name() method that fetches properties for only the specific datastore needed, reducing data transfer to ~10-20KB.

2. Add a 5-minute TTL cache for datastore name -> moref mappings in get_ds_ref_by_name(). Since volume creates are bursty and typically target the same datastores, this eliminates repeated vCenter queries for the lightweight name lookup on cache hits.

3. The host availability check still queries vCenter on every call to ensure we always have fresh data about which hosts are connected and not in maintenance mode.

Performance impact:

• First call: 1 lightweight query (names only) + 1 targeted query
• Subsequent calls (within 5 min): 1 targeted query only (cache hit)
• Original: 1 heavy query fetching all datastores with all host mounts

Change-Id: If9d46fe833418b67393535384af075a95f2ca4cb