store-spec->client-opts dissociates :role-arn entry

This commit modifies the store-spec->client-opts function so that, if
the argument map contains a :role-arn entry, the returned map does not
contain a :role-arn entry. This behavior is desired because it ensures
that the function is idempotent over multiple applications.
Merge pull request #1 from stevemkim/master

store-spec->client-opts dissociates :role-arn entry