Reorganized authentication implementation locations.

For paving the way into further authentication mechanism
implementations, the existing is reorganized.

Move src/sasl to src/authentication/cram_md5.
Rename src/tests/sasl_tests.cpp to src/tests/cram_md5_authentication_tests.cpp.
Adapt depending include paths.
Rename specific test implementations from SASL to CRAMMD5Authentication.
Rename sasl namespace to cram_md5 throughout all uses.
Add missing license blob to auxprop.cpp.

Review: https://reviews.apache.org/r/27122
Added CRAM-MD5 Test Authenticator module.

Introducing the CRAM-MD5 SASL test authenticator module based on the
former sasl/authenticator.

The former sasl/authenticator.hpp still remains as part of libmesos but
now is located at authentication/cram_md5.

Review: https://reviews.apache.org/r/26857
Integrated CRAM-MD5 Authenticator module.

Enables selecting a module based authenticator via the new
--authenticators flag for mesos master.

Additionally, all "> >" have been fixed towards ">>" in master.hpp and
master.cpp.

Review: https://reviews.apache.org/r/26859
Integrated CRAM-MD5 Authenticator module into tests.

The CRAM-MD5 authenticator module is now usable via mesos-tests.sh

Example usage:
bin/mesos-tests.sh \
--authenticators=org_apache_mesos_TestCRAMMD5Authenticator \
--modules="{\"libraries\": [{\"file\": \
\"src/.libs/libtestauthenticator.so\",\"modules\": [{\"name\": \
\"org_apache_mesos_TestCRAMMD5Authenticator\"}]}]}"

Review: https://reviews.apache.org/r/26860
Minor fix to balloon framework test.
Updated Modules protobuf to simplify JSON parsing.

Updates the --modules help messages accordingly.

Review: https://reviews.apache.org/r/27481
Updated --isolation help message for slave.

Now also mentions the --modules flag.

Review: https://reviews.apache.org/r/27560
First version of docs/modules.md.

With bits copied from
https://cwiki.apache.org/confluence/display/MESOS/Mesos+Modules+Developer+Guide.

Review: https://reviews.apache.org/r/27453