Enforce build with Java 11 for all by default

- Build with JDK11 and language levels (source and target) are now
enforced in .bazelrc, which depends on our own Java toolchain (defined
in tools/build/bazel/BUILD)
- Easymock bumped to v4.0.2 with support for Java 11 (minor API changes)
- The minimum required Bazel version is now 0.27.*

Change-Id: I0bb6dfe7fc1ea134b5a53dd249c369d58259688c
diff --git a/tools/build/bazel/BUILD b/tools/build/bazel/BUILD
index d819d63..cf20246 100644
--- a/tools/build/bazel/BUILD
+++ b/tools/build/bazel/BUILD
@@ -1,3 +1,17 @@
+load("@bazel_tools//tools/jdk:default_java_toolchain.bzl", "default_java_toolchain")
+
+# This is where we define the language source and target values passed to javac
+# when building ONOS. This toolchain should be used when invoking bazel build
+# (see onos/.bazelrc)
+default_java_toolchain(
+    name = "default_toolchain",
+    source_version = "11",
+    target_version = "11",
+    visibility = [
+        "//visibility:public",
+    ],
+)
+
 py_binary(
     name = "osgi_feature_bundler",
     srcs = ["osgi_feature_bundler.py"],