Add region-specific host constants.

Some users need to log in to specific regions only.  This PR exposes
host constants that make this easier, like so:

    import tiledb.cloud
    from tiledb.cloud.region import AWS
    tiledb.cloud.login(host=AWS.US_EAST_1)

As of today, we support five AWS regions.

src/tiledb/cloud/client.py

@@ -78,7 +78,8 @@ def login(
     :param token: api token for login
     :param username: username for login
     :param password: password for login
-    :param host: host to login to
+    :param host: host to login to. the tiledb.cloud.regions module contains
+        region-specific host constants.
     :param verify_ssl: Enable strict SSL verification
     :param no_session: don't create a session token on login,
         store instead username/password

src/tiledb/cloud/region/__init__.py

@@ -0,0 +1,19 @@
+class AWS:
+    """
+    The AWS cloud regions that are currently supported.
+
+    US_EAST_1 = North America, United States (Virginia)
+    US_WEST_1 = North America, United States (California)
+    EU_WEST_1 = Europe, Ireland
+    EU_WEST_2 = Europe, London
+    AP_SOUTHEAST_1 = Asia, Singapore
+    """
+
+    US_EAST_1 = "https://us-east-1.aws.api.tiledb.com"
+    US_WEST_1 = "https://us-west-1.aws.api.tiledb.com"
+    EU_WEST_1 = "https://eu-west-1.aws.api.tiledb.com"
+    EU_WEST_2 = "https://eu-west-2.aws.api.tiledb.com"
+    AP_SOUTHEAST_1 = "https://ap-southeast-1.aws.api.tiledb.com"
+
+
+__all__ = ("AWS",)