Attach multiple FileStores to a cluster.

[isaacabraham]

This is an important feature in that it would allow you to easily create a cluster and have it connect to e.g. customer data store without necessarily writing any MBrace-specific data to it, by using the primary storage account for MBrace-specific data and secondaries only for actual user data to operate over.

This functionality would probably be dependent on #153 - I know that you can manually override the FileStore on a per-query basis but having the ability to connect multiple file stores and have them permanently there would be very useful.

In HDInsight, you specify these at creation time - the primary account acts as the default one and does not need to be specified in queries, whereas secondary accounts need to be prefixed with the account name in the WASB path.

This is one approach that could be adopted; a second option would be to allow adding / removing accounts post-creation of a cluster.