Added --isolation flag for tests.

This is useful to test custom isolation modules with the test suite.
The user can specify the available modules with --modules and then he
can specify the isolator module with the --isolation flag.
With these flags, the test suite will run all tests with the specified
isolator modules.

Review: https://reviews.apache.org/r/26797
Added support for module-specific command-line parameters.

The Modules protobuf is enhanced to support key-value parameters based
on mesos::Parameter and mesos::Parameters.

Bumped Module API version since we modified the signature of the
create() method.

Review: https://reviews.apache.org/r/27051
Support both 1.8+ and earlier versions of svn library

Review: https://reviews.apache.org/r/27243