Install injection libraries with agent install script

[randomanderson]

(resubmission of #36)

This PR adds the ability to install autoinstrumentation libraries after setting environment variables.

Variable name	Description
DD_APM_INSTRUMENTATION_ENABLED	Install host and docker injection unless overridden with a more specific env variable
DD_APM_HOST_INJECTION_ENABLED	Installs or prevents installation of host injection (via LD_PRELOAD)
DD_APM_DOCKER_INJECTION_ENABLED	Installs or prevents installation of docker injection ( via daemon.json )
DD_APM_LIBRARIES	Space separated list of which languages to install support for. Valid values are js, java, python, dotnet, all. Defaults to all if host or docker injection is enabled
DD_NO_AGENT_INSTALL	If set, don't install the datadog agent. This is useful for the container injection only case. The check is for the existence of the variable, not any particular value. This mirrors DD_INSTALL_ONLY

• The script does not verify that all combinations of new and existing variables make sense together (for instance DD_NO_AGENT_INSTALL and DD_AGENT_MINOR_VERSION)
• I removed the Installing the $nice_flavor package log statement because it is redundant with Installing package(s) ... line that follows and is misleading in some cases.

Testing

I added few basic checks to the test suite. Only host injection could be tested because of the docker-in-docker issue.

[bkabrda]

LGTM and the CI is passing now. Thanks a lot for the PR, I'm merging it.