Set a global -Xmx default for JVMs

Note that ZincCompile retains a much higher default.

Also note that all current Pants repos override this
in pants.ini anyway, so this is primarily so that new
users can get up and running with some sort of sensible
default.

256M was chosen as a compromise between having to little
memory to get anything done and having several nailguns
chew up all RAM on the system.

Testing Done:
CI passes: https://travis-ci.org/pantsbuild/pants/builds/123511811

Reviewed at https://rbcommons.com/s/twitter/r/3705/

src/python/pants/backend/jvm/subsystems/jar_tool.py

@@ -14,10 +14,6 @@
 class JarTool(JvmToolMixin, Subsystem):
   options_scope = 'jar-tool'
 
-  @classmethod
-  def get_jvm_options_default(cls, bootstrap_option_values):
-    return ['-Xmx64M']
-
   @classmethod
   def register_options(cls, register):
     super(JarTool, cls).register_options(register)

src/python/pants/backend/jvm/subsystems/jvm.py

@@ -23,7 +23,7 @@ class JVM(Subsystem):
 
   # Broken out here instead of being inlined in the registration stanza,
   # because various tests may need to access these.
-  options_default = []
+  options_default = ['-Xmx256m']
 
   @classmethod
   def register_options(cls, register):

src/python/pants/backend/jvm/subsystems/jvm_tool_mixin.py

@@ -58,7 +58,7 @@ def get_jvm_options_default(cls, bootstrap_option_values):
                                     See src/python/pants/options/options_bootstrapper.py for
                                     details.
     """
-    return []
+    return ['-Xmx256m']
 
   @classmethod
   def register_options(cls, register):