Fix specifying tag on CodeBuild repository image to use (#377)

**Why?**
As reported in issue #374, the schema validation looks for a dictionary
of tags when a `repository_arn` is set. While the rest of the code, tests, and
documentation talk about a tag string.

**What?**
This change requests updates the schema to fix this issue.

docs/providers-guide.md

@@ -230,6 +230,7 @@ Provider type: `codebuild`.
   > pipeline to consume a custom image if required.
   > Along with `repository_arn`, we also support a `tag` key which can be used
   > to define which image should be used (defaults to `latest`).
+  > An example of this setup is provided [here](https://github.com/awslabs/aws-deployment-framework/blob/master/docs/user-guide.md#custom-build-images).
 - *size* *(String)* **(small|medium|large)** - default: `small`.
   > The Compute type to use for the build, types can be found
   > [here](https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-compute-types.html).

docs/user-guide.md

@@ -314,7 +314,7 @@ pipelines:
         provider: codebuild
         image:
           repository_arn: arn:aws:ecr:region:012345678910:repository/test
-          tag: latest #optional (and also defaults to latest)
+          tag: latest # optional (defaults to latest)
     targets:
       - ...
 ```

src/lambda_codebase/initial_commit/bootstrap_repository/adf-build/shared/schema_validation.py

@@ -99,7 +99,7 @@
 # CodeBuild
 CODEBUILD_IMAGE_PROPS = {
     "repository_arn": str, # arn:aws:ecr:region:012345678910:repository/test
-    Optional("tags"): dict,
+    Optional("tag"): str,  # defaults to latest
 }
 CODEBUILD_PROPS = {
     Optional("image"): Or(str, CODEBUILD_IMAGE_PROPS),