gravity install waits for all nodes to join

[adamlamar]

When using a flavor that specifies multiple nodes, the current behavior of gravity install is to timeout after some time if a node doesn't join the cluster quickly enough.

In the case of an interactive install where a user is manually joining nodes to the cluster, it would be helpful if gravity install waited indefinitely until the required cluster nodes joined. This behavior also seems helpful in the case where a large number of nodes need to join.

Proposal:

• Add a flag like --wait or --wait-for-joins that causes the initial gravity install command to wait indefinitely for all nodes to join