Resolve default package definitions if extact version is unavailable.

Old behavior can be reverted by setting up a dependency_resolvers_conf.xml with the following contents

```
<dependency_resolvers>
  <tool_shed_packages />
  <galaxy_packages />
</dependency_resolvers>
```

The new behavior corresponds to the following dependency_resolvers_conf.xml contents:

```
<dependency_resolvers>
  <tool_shed_packages />
  <galaxy_packages />
  <galaxy_packages versionless="true" />
</dependency_resolvers>
```

Still think galaxy_packages should come before tool_shed_packages in resolution order so that deployers can fix broken tool shed installs with manual installs or deploy optimized versions of packages without having to mess with a dependency_resolvers_conf.xml file.

lib/galaxy/tools/deps/__init__.py

@@ -76,6 +76,7 @@ class DependencyManager( object ):
         return [
             ToolShedPackageDependencyResolver(self),
             GalaxyPackageDependencyResolver(self),
+            GalaxyPackageDependencyResolver(self, versionless=True),
         ]
 
     def __parse_resolver_conf_xml(self, tree):

test/unit/test_tool_deps.py

@@ -42,9 +42,10 @@ def test_tool_dependencies():
         dependency = dm.find_dep( "dep1", None )
         assert dependency.version == "2.0"
 
-        ## Test default will not be fallen back upon by default
+        ## Test default resolve will be fall back on default package dependency
+        ## when using the default resolver.
         dependency = dm.find_dep( "dep1", "2.1" )
-        assert dependency == INDETERMINATE_DEPENDENCY
+        assert dependency.version == "2.0"  # 2.0 is defined as default_version
 
 
 TEST_REPO_USER = "devteam"