Fixing several bugs in assemble and updating tests

Bugfixes:
* the remote_src param was not being converted to a boolean correctly,
  resulting in it never being used by the module as the default behavior
  was remote_src=True (issue #5581)
* the remote_src param was not listed in the generic file params, leading
  to a failure when the above bug regarding remote_src was fixed
* the delimiter should always end with a newline to ensure that the file
  fragments do not run together on one line

Fixes #5581

lib/ansible/module_utils/basic.py

@@ -111,6 +111,7 @@ FILE_COMMON_ARGUMENTS=dict(
     content = dict(),
     backup = dict(),
     force = dict(),
+    remote_src = dict(), # used by assemble
 )
 
 def get_platform():