FLINK-2213: Makes the number of vcores per YARN container configurable.

This closes #1588

docs/setup/config.md

@@ -240,6 +240,8 @@ definition. This scheme is used **ONLY** if no other scheme is specified (explic
 
 	yarn.application-master.env.LD_LIBRARY_PATH: "/usr/lib/native"
 
+- `yarn.containers.vcores` The number of virtual cores (vcores) per YARN container. By default, the number of `vcores` is set to the number of slots per TaskManager, if set, or to 1, otherwise. 
+
 - `yarn.taskmanager.env.` Similar to the configuration prefix about, this prefix allows setting custom environment variables for the TaskManager processes.
 
 - `yarn.application-master.port` (Default: 0, which lets the OS choose an ephemeral port) With this configuration option, users can specify a port, a range of ports or a list of ports for the  Application Master (and JobManager) RPC port. By default we recommend using the default value (0) to let the operating system choose an appropriate port. In particular when multiple AMs are running on the  same physical host, fixed port assignments prevent the AM from starting.

flink-core/src/main/java/org/apache/flink/configuration/ConfigConstants.java

@@ -242,6 +242,11 @@ public final class ConfigConstants {
 
 	// ------------------------ YARN Configuration ------------------------
 
+	/**
+	 * The vcores exposed by YYARN.
+	 */
+	public static final String YARN_VCORES = "yarn.containers.vcores";
+
 	/**
 	 * Percentage of heap space to remove from containers started by YARN.
 	 */