update doc since validation has been added to bcd_stack_id variable (#6)

* update doc since validation has been added to bcd_stack_id variable

md/known_issues.md

 # Known issues
 
-- Using '.' or '-' in `bcd_stack_id` will cause errors: the created EC2 instances won't be able to be retrieved for stack deployment.
 - On a BCD controller we can manage only one Terraform stack at a time.

md/scenarios.md

@@ -16,7 +16,7 @@ Some parameters are mandatory for general provisioning and test description.
 
 |Name |Mandatory|Default|Description|Sample|
 |-|-|-|-|-|
-|bcd_stack_id|Y| |Id of the test. Used for tests identification and test results export directory|jenkins-test|
+|bcd_stack_id|Y| |Bonita stack identifier used to group machines by stack membership. For AWS deployments the characters allowed are limited to letters, numbers and underscore.|uswest2_test|
 |configuration|Y | |Configuration filename. Initialize main variables. Choose between aws_vars, vagrant_vars|aws_vars|
 |ansible_ssh_user|N|ubuntu|SSH user|ubuntu|
 |ansible_private_key_file|Y |-|Path for the private SSH key.|~/.ssh/jenkins_ansible_us-west-2.pem|