Adds import capability to the `aws_codecommit_repository` resource.
Also fixes an issue in the Read function where both `description` and `resource_name` attributes weren't being updated and set in the Schema.
Fixes: #13559
```
$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSCodeCommitRepository"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/12 10:14:44 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSCodeCommitRepository -timeout 120m
=== RUN TestAccAWSCodeCommitRepository_importBasic
--- PASS: TestAccAWSCodeCommitRepository_importBasic (16.11s)
=== RUN TestAccAWSCodeCommitRepository_basic
--- PASS: TestAccAWSCodeCommitRepository_basic (14.97s)
=== RUN TestAccAWSCodeCommitRepository_withChanges
--- PASS: TestAccAWSCodeCommitRepository_withChanges (26.71s)
=== RUN TestAccAWSCodeCommitRepository_create_default_branch
--- PASS: TestAccAWSCodeCommitRepository_create_default_branch (14.34s)
=== RUN TestAccAWSCodeCommitRepository_create_and_update_default_branch
--- PASS: TestAccAWSCodeCommitRepository_create_and_update_default_branch (27.90s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 100.071s
```
In AWS codecommit the default branch must have a value unless there are
no branches created, in which case it is not possible to set it to any value.
We query the existing branches and do not update the default branch
if there are none defined remotely.
This solves the issue of the initial creation of the repository with a
resource with `default_branch` defined.
Jake Champlin