Branches-1 precommit and nightly yetus jobs are using jdk8 for jdk7 jobs

branch-1 nightly job is failing while trying to do our yetus shaded jar test because the enforcer plugin complains about a jdk8 jdk-tools jar being present.

looking at the yetus footer, it's because jdk8 is being used instead of jdk7:

[2019-08-30T13:30:34.353Z] | maven | version: Apache Maven 3.0.5 |
[2019-08-30T13:30:34.353Z] | Default Java | 1.8.0_222 |
 

Same thing in the footer of the jdk7 build (which passes, even though it shouldn't)

[2019-08-30T15:43:29.103Z] | maven | version: Apache Maven 3.0.5 |
[2019-08-30T15:43:29.103Z] | Default Java | 1.8.0_222 |

Both builds appear to be properly running in Docker mode.