Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/aws/exercises/elasticache/exercise.md

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+## AWS ElastiCache
+
+### Objectives
+
+1. Create ElastiCache Redis
+  * Instance type should be "cache.t2.micro"
+  * Replicas should be 0

topics/aws/exercises/elasticache/solution.md

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+## AWS ElastiCache
+
+### Objectives
+
+1. Create ElastiCache Redis
+  * Instance type should be "cache.t2.micro"
+  * Replicas should be 0
+
+### Solution
+
+#### Console
+
+1. Go to ElastiCache service
+2. Click on "Get Started Now"
+3. Choose "Redis"
+4. Insert a name and description
+5. Choose "cache.t2.micro" an node type
+6. Set number of replicas to 0
+7. Create new subnet group
+8. Click on "Create"