Revert "Use LTS version of Ubuntu in Dockerfiles (#55327)"

This reverts commit cc09e24.

Author: pugnascotia

test/fixtures/azure-fixture/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:19.04
 RUN apt-get update -qqy
 RUN apt-get install -qqy openjdk-12-jre-headless
 ENTRYPOINT exec java -classpath "/fixture/shared/*" fixture.azure.AzureHttpFixture 0.0.0.0 8091 container

test/fixtures/gcs-fixture/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:19.04
 
 RUN apt-get update -qqy
 RUN apt-get install -qqy openjdk-12-jre-headless
@@ -14,4 +14,4 @@ ENV GCS_FIXTURE_TOKEN=${token}
 ENTRYPOINT exec java -classpath "/fixture/shared/*" \
     fixture.gcs.GoogleCloudStorageHttpFixture 0.0.0.0 "$GCS_FIXTURE_PORT" "$GCS_FIXTURE_BUCKET" "$GCS_FIXTURE_TOKEN"
 
-EXPOSE $port
+EXPOSE $port

test/fixtures/s3-fixture/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:19.04
 
 RUN apt-get update -qqy
 RUN apt-get install -qqy openjdk-12-jre-headless

x-pack/test/smb-fixture/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:16.04
 RUN apt-get update -qqy && apt-get install -qqy samba ldap-utils
 ADD . /fixture
 RUN chmod +x /fixture/src/main/resources/provision/installsmb.sh