.yml files with {cipher} tokens, without quotes, breaks the config service

# Problem

If a configuration property starts with `{cipher}`, but without single/double-quotes, the configuration service breaks. This is specific to `.yml` files.

Example at https://github.com/marcellodesales/config-repo/commit/3243ff4d7c853418bd73bfe349aef848599f5ca2

# Config Server API Output

```
$ curl config-server:8888/app/default
Error: Failed to load property source from location 'file:/tmp/config-repo-2392906881014410043/application.yml'
```

# Config Server Error Logs

```
config-server_1 | ... 87 common frames omitted
config-server_1 |
config-server_1 | 2017-12-07 07:50:05.983 ERROR 1 --- [nio-8888-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalStateException: Failed to load property source from location 'file:/tmp/config-repo-2392906881014410043/application.yml'] with root cause
config-server_1 |
config-server_1 | org.yaml.snakeyaml.parser.ParserException: while parsing a block mapping
config-server_1 | in 'reader', line 5, column 3:
config-server_1 | password: {cipher}AQBdTe9vjgOqAN ...
config-server_1 | ^
config-server_1 | expected , but found Scalar
config-server_1 | in 'reader', line 5, column 21:
config-server_1 | password: {cipher}AQBdTe9vjgOqANyfi+Ke5HYmhFJnuvsG ...
config-server_1 | ^
config-server_1 |
config-server_1 | at org.yaml.snakeyaml.parser.ParserImpl$ParseBlockMappingKey.produce(ParserImpl.java:569) ~[snakeyaml-1.17.jar!/:na]
config-server_1 | at org.yaml.snakeyaml.parser.ParserImpl.peekEvent(ParserImpl.java:157) ~[snakeyaml-1.17.jar!/:na]
config-server_1 | at org.yaml.snakeyaml.parser.ParserImpl.checkEvent(ParserImpl.java:147) ~[snakeyaml-1.17.jar!/:na]
config-server_1 | at org.yaml.snakeyaml.composer.Composer.composeMappingNode(Composer.java:224) ~[snakeyaml-1.17.jar!/:na]
```

# Proposed Solution

* Verify if the value of keys contains the valid tokens for secrets such as `{cipher}` and `{secret}` contains quotes (single or double) to make sure secrets are correctly set.

Start with the repository's YAML validation path and reproduce the unquoted `{cipher}` value from the issue in an `.yml` file. Check how `{cipher}` and `{secret}` values are currently handled by the script or pre-receive hook. Done means unquoted secret tokens are detected or rejected while quoted values remain valid.

Written by the indexing model from the issue text.