* [issue-30] Optional elasticache_subnet_group ID

Add optional variable for the Elasticache subnet group ID so we can 
create the replication group in an already existing subnet group.

* Update main.tf
Co-Authored-By: aknysh <aknysh@users.noreply.github.com>

* Update variables.tf
Co-Authored-By: aknysh <aknysh@users.noreply.github.com>

* Update variables.tf
Co-Authored-By: aknysh <aknysh@users.noreply.github.com>

* Update main.tf
Co-Authored-By: aknysh <aknysh@users.noreply.github.com>

* Address comments, update LICENSE, update descriptions, update `README`

* Update example

* Update README