update stacks-node.go to allow for followers to be run

[wileyj]

current code makes seq 1 the master node, running btc in regtest.
all other instances come up (as well as seq 1) as a miner.

it would be good to have a few followers in the mix of hosts during the test. since we can't say a test will be run with a predefined set of nodes, one idea is to set aside a % of the total instances as follower nodes.
for example:

• if we run with 10 instances -> 50% of them will be followers, 1 as master, and the rest as miners.
• if we run with 50 instances -> 75% of them will be followers, 1 as master, and the rest as miners.

@reedrosenbluth