Add support for custom resource names

[gari-bergara-jt]

Since the current replication group, subnet group, security group and parameter group names are not configurable via variable, there's no way to apply any other naming convention than what the module uses by default to them.

The idea of this PR is to make those names configurable via the following variables, which are empty by default and are not required:

• elasticache_parameter_group_name
• elasticache_replication_group_id
• elasticache_subnet_group_name
• elasticache_security_group_name

[gari-bergara-jt]

@antonbabenko would you mind taking a look whenever you have time? Thanks!