By default, don't specify cert alias for migrator's lightblue client

[paterczm]

At some point, lightblue-client was updated to actually use the alias configuration in lightblue-client.properties. Since it was set incorrectly, migrator is now failing after it was updated to use new lightblue-client. This change will make it use not use an alias by default (it that case it expects only on cert in the store).