[packaging] support centOS smoke testing

[v1v]

What

Let's support centOS for testing the rpm package in different PHP versions:

• 7.2
• 7.3
• 7.4

Tests

$ rm -rf build ; PHP_VERSION=7.3 make -C packaging  rpm rpm-install
...
{:timestamp=>"2020-07-23T10:40:39.382287+0000", :message=>"Package version '0.1-preview' includes dashes, converting to underscores", :level=>:warn}
{:timestamp=>"2020-07-23T10:40:40.268219+0000", :message=>"Created package", :path=>"build/packages/apm-agent-php-0.1_preview-1.noarch.rpm"}
Creating sha512sum ...
...
+ composer run-script run_component_tests_standalone_envs
> ELASTIC_APM_TESTS_APP_CODE_HOST_KIND=CLI_script phpunit -c phpunit_component_tests.xml
PHPUnit 8.5.8 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.20
Configuration: /src/phpunit_component_tests.xml

...                                                                 3 / 3 (100%)

Time: 5.4 seconds, Memory: 10.00 MB

OK (3 tests, 65 assertions)
> ELASTIC_APM_TESTS_APP_CODE_HOST_KIND=CLI_builtin_HTTP_server phpunit -c phpunit_component_tests.xml
PHPUnit 8.5.8 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.20
Configuration: /src/phpunit_component_tests.xml

...                                                                 3 / 3 (100%)

Time: 5.49 seconds, Memory: 10.00 MB

OK (3 tests, 65 assertions)

[apmmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Pull request #99 updated]

• Start Time: 2020-07-23T11:24:35.014+0000

• Duration: 9 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	78
Skipped	0
Total	78

[SergeyKleyman]

LGTM