[WIP] 337 fix creating replication with source and target strings

[emlaver]

What

• Fix the null error that occurs when creating a replicator with source and target strings.
• Properly handle repl_id when only source and target parameters exist
• Decide on how source and target dictionaries should be passed in (see below)

How

• Added null check on target_db before calling .admin_party. Also set 'user_ctx' if the database object's 'creds' exists.
• WIP: improve the logic to set repl_id. When source and target parameters exist, the passed in repl_id value is set in source_db. I've added logic to set repl_id = source_db if source and target exist and source_db is a string.

Testing

• Added unit tests to verify that source and target string parameters work as expected
• If source or target url do not contain credentials, replication_state should equal error since the user is unauthorized

Issues

fixes #337

TODO: Remove replicator doc that currently exists after tests finish.

TODO: How should source and target dictionaries be passed in? If this below is the expected way, then this will also require fixing as it's not properly handled:

replication_doc = Replicator(client).create_replication(
    {'source': 'src_db', 'target': 'tgt_db'}
)

[emlaver]

@smithsz @ricellis from our slack discussion, we will be removing the source and target optional parameters.