Streaming node add

Rather than waiting for all spot instances and then for all instances ssh to be up prior to starting adding the nodes to a cluster, adds the node as they come ready.

+ May be useful when adding many nodes at once. The greater the amount of added nodes is, the greater the probability this will be beneficial is.
+ No difference when adding a single node.

Note: This is based from 2 bugfix pull requests, namely https://github.com/jtriley/StarCluster/pull/433 and https://github.com/jtriley/StarCluster/pull/433