audit docker packaging tests

[graphaelli]

With the recent move to own docker packaging for apm-server in #1543, ensure that all of the tests covered by apm-server-docker are also covered now - some are already accounted for in the package tests.

[graphaelli]

all of https://github.com/elastic/apm-server-docker/blob/master/tests/test_config.py:

• test_config_file_passes_config_test - apm-server -configtest exits 0 (should actually be apm-server test config)
• test_apm_server_listens_on_all_interfaces - apm-server.host == '0.0.0.0:8200'
• test_elasticsearch_output_points_to_elasticsearch_host - output.elasticsearch.hosts == ['elasticsearch:9200']

from https://github.com/elastic/apm-server-docker/blob/master/tests/test_files.py:

• test_suid_bit_removed - find / -xdev -perm -4000 returns 0 and no files

all of https://github.com/elastic/apm-server-docker/blob/master/tests/test_labels.py

• license check in particular

[jalvz]

@graphaelli where do we are here? needs review?

[graphaelli]

looks like I just forgot to close this, thanks