Fix --force for unversionned requirements (ansible#23391)

In current stable (2.2), ansible galaxy install --force do erase
a role, even if the version is not set. This commit should restore
that specific behavior, in accordance to people reports:
  ansible#11266 (comment)

It was also the behavior planned in the initial discussion:
"if you're not fixing versions in your roles file, then it's fine
to expect that the role will be reinstalled each time you run
ansible-galaxy install.", cf ansible#12904

lib/ansible/cli/galaxy.py

@@ -389,8 +389,9 @@ def execute_install(self):
                                         (role.name, role.install_info['version'], role.version or "unspecified"))
                         continue
                 else:
-                    display.display('- %s is already installed, skipping.' % str(role))
-                    continue
+                    if not force:
+                        display.display('- %s is already installed, skipping.' % str(role))
+                        continue
 
             try:
                 installed = role.install()