Fix usage of depends_on condition attribute (#95)

Author: davidkopp

content/en/docs/measuring/usage-scenario.md

@@ -81,7 +81,8 @@ services:
   gcb-wordpress-apache:
     # ...
     depends_on:
-      gcb-wordpress-mariadb: service_healthy
+      gcb-wordpress-mariadb:
+        condition: service_healthy
   gcb-wordpress-dummy:
     # ...
     depends_on:
@@ -101,10 +102,10 @@ services:
     - `ports:` **[int:int]** *(optional)*
       + Docker container portmapping on host OS to be used with `--allow-unsafe` flag.
     - `depends_on:` **[list|dict]** *(optional)*
-      + Can either be an list of services names on which the service is dependent. It affects the startup order and forces the dependency to be "ready" before the service is started. 
-      + Or it can be an dict where each key represents a service as a string. The string then can have two values:
-          * `service_healthy`: Will wait for the container until the docker *healthcheck* returns *healthy*.
-          * `service_started`: Similar to the list syntax this will enforce a starting order and just wait until the container has been created.
+      + Can either be an list of services names on which the service is dependent. It affects the startup order and forces the dependency to be started (container state = "running") before the service is started.
+      + Or it can be an dict where you can specify a condition. The `condition` can have two values:
+        * `service_healthy`: Will wait for the container until the docker *healthcheck* returns *healthy*.
+        * `service_started`: Similar to the list syntax -> this will enforce a starting order and just wait until the container state is "running".
     - `setup-commands:` **[list]** *(optional)*
       + List of commands to be run before actual load testing. Mostly installs will be done here. Note that your docker container must support these commands and you cannot rely on a standard linux installation to provide access to /bin
     - `volumes:` **[list]**  *(optional)*
@@ -113,7 +114,7 @@ services:
       + The networks to put the container into. If no networks are defined throughout the `usage_scenario.yml` the container will be put into the default network will all others in the file.
     - `healthcheck:` **[dict]** *(optional)*
       + Please see the definition of these arguments and how healthcheck works in the official docker compose definition. We just copy them over: [Docker compose healthcheck specification](https://docs.docker.com/compose/compose-file/compose-file-v3/#healthcheck)
-      + `test:` **[str|list]**         
+      + `test:` **[str|list]**
       + `interval:` **[str]**
       + `timeout:` **[str]**
       + `retries:` **[integer]**