Devenv: update mysql_tests and postgres_tests blocks for allowing dynamically…

Devenv: update mysql_tests and postgres_tests blocks for allowing dynamically change of underlying docker image (#29525)

* Devenv: allow dynamically change mysql_tests docker image

* Devenv: allow dynamically change postgres_tests docker image

devenv/docker/blocks/mysql_tests/.env

+mysql_version=5.6

devenv/docker/blocks/mysql_tests/Dockerfile

-FROM mysql:5.6
+ARG mysql_version=5.6
+FROM mysql:${mysql_version}
 ADD setup.sql /docker-entrypoint-initdb.d
 CMD ["mysqld"]

devenv/docker/blocks/mysql_tests/docker-compose.yaml

   mysqltests:
     build:
       context: docker/blocks/mysql_tests
+      args:
+        - mysql_version=${mysql_version}
     environment:
       MYSQL_ROOT_PASSWORD: rootpass
       MYSQL_DATABASE: grafana_tests

devenv/docker/blocks/postgres_tests/.env

+postgres_version=9.3

devenv/docker/blocks/postgres_tests/Dockerfile

-FROM postgres:9.3
+ARG postgres_version=9.3
+FROM postgres:${postgres_version}
 ADD setup.sql /docker-entrypoint-initdb.d
 CMD ["postgres"]

devenv/docker/blocks/postgres_tests/docker-compose.yaml

   postgrestest:
     build:
       context: docker/blocks/postgres_tests
+      args:
+        - postgres_version=${postgres_version}
     environment:
       POSTGRES_USER: grafanatest
       POSTGRES_PASSWORD: grafanatest