Add optional targets_list argument

[TokRa13]

https://www.pivotaltracker.com/story/show/170425434

Because we moved tg_attachements from terraform to deploy-tools now we need to manually add targets to target groups when adding new instances.

With this changes we can specify a list of instances instead.

[ptqa]

I don't really understand the use case for that

[TokRa13]

I don't really understand the use case for that

If we add new instances and run deploy from application repo (e.g. simplepractice) - there will be running docker containers on those new hosts but they will not be added as a target to appropriate target groups.

With these changes we can either run manually to change target group like blue_green_switch simplepractice blue 8190 i-0c21ad023330d207b,i-051d6a2bb65c3564f
or change applications deploy code to fetch instances-id automatically as in this example: https://github.com/simplepractice/simplepractice-events/pull/226

[TokRa13]

Or we can add targets manually of course... but why do it if we can automate it