JCLOUDS-299. implement multi-region support in openstack-swift

Implementation of `BlobStoreContext` which allows you to employ multiple regions.

Example.

```java
ctx = contextBuilder.buildView(RegionScopedBlobStoreContext.class);

Set<String> regionIds = ctx.configuredRegions();

// isolated to a specific region
BlobStore texasBlobStore = ctx.blobStoreInRegion("US-TX");
BlobStore virginiaBlobStore = ctx.blobStoreInRegion("US-VA");
```

Note similar methods are also exposed for blob signer and async blobstore.