Allow running eclipselsp as installed by system package on GNU/Linux (#2523)

* Search eclipselsp jar and config files within system package path
* Allow setting an alternate eclipselsp configuration directory
* Add test for ale_java_eclipselsp_config_path

test/command_callback/test_eclipselsp_command_callback.vader

@@ -85,3 +85,20 @@ Execute(The eclipselsp callback should allow custom executable):
     \]
   AssertLinter '/bin/foobar', join(cmd, ' ')
 
+Execute(The eclipselsp callback should allow custom configuration path):
+  let b:ale_java_eclipselsp_config_path='/home/config'
+  let cmd = [ ale#Escape('java'),
+    \ '-Declipse.application=org.eclipse.jdt.ls.core.id1',
+    \ '-Dosgi.bundles.defaultStartLevel=4',
+    \ '-Declipse.product=org.eclipse.jdt.ls.core.product',
+    \ '-Dlog.level=ALL',
+    \ '-noverify',
+    \ '-Xmx1G',
+    \ '-jar',
+    \ '',
+    \ '-configuration',
+    \ b:ale_java_eclipselsp_config_path,
+    \ '-data',
+    \ ''
+    \]
+  AssertLinter 'java', join(cmd, ' ')