Cherry-pick #14587 to 7.x: Add target to build images from supported-versions.yml file

[jsoriano]

Cherry-pick of PR #14587 to 7.x branch. Original message:

Add mage targets intended to be used mostly by CI to prebuild
and push images used by integration and E2E tests. It leverages
existing docker compose scenarios, and build all the images in
the docker-compose.yml file associated to each
supported-versions.yml file.

To find the associated docker-compose.yml file, it follows similar
behaviour we have in other places, it starts descending directories
till it finds a directory with a docker-compose.yml file.

Usage example:

$ mage compose:buildSupportedVersions
2019/11/18 17:10:31 Found Elastic Beats dir at /home/jaime/gocode/src/github.com/elastic/beats
>> compose: Building docker images for supported versions
>> compose: Using compose file module/mysql/docker-compose.yml
>> compose: Building images for variant map[MYSQL_VARIANT:mysql MYSQL_VERSION:5.7.12]
>> compose: Building images for variant map[MYSQL_VARIANT:mysql MYSQL_VERSION:8.0.13]
>> compose: Building images for variant map[MYSQL_VARIANT:percona MYSQL_VERSION:5.7.24]
>> compose: Building images for variant map[MYSQL_VARIANT:percona MYSQL_VERSION:8.0.13-4]
>> compose: Building images for variant map[MYSQL_VARIANT:mariadb MYSQL_VERSION:10.2.23]
>> compose: Building images for variant map[MYSQL_VARIANT:mariadb MYSQL_VERSION:10.3.14]
>> compose: Building images for variant map[MYSQL_VARIANT:mariadb MYSQL_VERSION:10.4.4]
>> compose: OK

For push:

$ mage compose:pushSupportedVersions
2019/11/18 17:15:30 Found Elastic Beats dir at /home/jaime/gocode/src/github.com/elastic/beats
>> compose: Pushing docker images for supported versions
>> compose: Using compose file module/mysql/docker-compose.yml
>> compose: Pushing images for variant map[MYSQL_VARIANT:mysql MYSQL_VERSION:5.7.12]
...

Related to #14520 and #14355.