This commit changes where the REST API specs are read. The REST API
spec is required to run the REST YAML tests and are currently copied
to every project that has REST YAML tests.

The REST API specs are now copied to the build directory of the root
gradle project. Since each module or plugin (including all x-pack plugins)
can contribute to the REST API spec, they too are copied to the common
directory. The result is a single directory created during the build which
includes the full REST API spec. Configuration is added to each project
that has a RestIntegTestTask to enable ensure that the common directory is
created when needed.

The numerous places where this spec was copied around has now been removed
in favor of the new common directory, which is the full spec and is part
of the test classpath.

Additionally, to enable plugin developers, a new plugin copy-rest-api-spec
has been introduced that allows the specs to be copied locally
from the jar. This is plugin is also used for the BWC tests which need
to copy the tests for the spec to the local resource directory.