allow traffic inside security group

[kevcube]

boolean flag to enable intra-security group communication.

why

• we create our app infrastructure beginning with the database, then we use the output security group ID that gets created by Postgres as an input in our application, to give it database access
• previously what we've done is create the database, create the app, and add the app security group to the database's inputs, but this makes circular dependencies because our app depends on other outputs of the database

[Gowiem]

/test all

[Gowiem]

@kevcube mind sharing where this is necessary in your PR description? That "Why" is helpful to understanding if this should be merged.

[kevcube]

@Gowiem added! I created my PR from GitHub.dev and it doesn't add any template 😵‍💫

[Gowiem]

/test all

This Pull Request has been updated, so we're dismissing all reviews.

[kevcube]

@Gowiem fixed

[Gowiem]

/test all

[Gowiem]

@kevcube Thanks for the contribution! Released as https://github.com/cloudposse/terraform-aws-rds-cluster/releases/tag/1.2.0