Salt backend: Implement glob or compound matchers

Closes pytest-dev#17

doc/source/backends.rst

@@ -54,10 +54,16 @@ salt
 The salt backend use the `salt python client API
 <http://docs.saltstack.com/en/latest/ref/clients/>`_ and can be used from the salt-master server::
 
+    $ testinfra --connection=salt # equivalent to --hosts='*'
     $ testinfra --connection=salt --hosts=minion1,minion2
+    $ testinfra --connection=salt --hosts='web*'
+    $ testinfra --connection=salt --hosts=G@os:Debian
 
 Testinfra will use the salt connection channel to run commands.
 
+Host can be seleted using `glob` or `compound matchers
+<https://docs.saltstack.com/en/latest/topics/targeting/compound.html>`_.
+
 
 .. _ansible connection backend:
 

testinfra/backend/salt.py

@@ -35,12 +35,16 @@ def __init__(self, host, *args, **kwargs):
         self._client = None
         super(SaltBackend, self).__init__(self.host, *args, **kwargs)
 
+    @staticmethod
+    def _check_salt():
+        if not HAS_SALT:
+            raise RuntimeError(
+                "You must install salt package to use the salt backend")
+
     @property
     def client(self):
         if self._client is None:
-            if not HAS_SALT:
-                raise RuntimeError(
-                    "You must install salt package to use the salt backend")
+            self._check_salt()
             self._client = salt.client.LocalClient()
         return self._client
 
@@ -57,3 +61,22 @@ def run_salt(self, func, args=None):
                 "Error while running %s(%s): %s. Minion not connected ?" % (
                     func, args, out))
         return out[self.host]
+
+    @classmethod
+    def get_hosts(cls, host, **kwargs):
+        if host is None:
+            host = "*"
+        if any([c in host for c in "@*[?"]):
+            cls._check_salt()
+            client = salt.client.LocalClient()
+            if "@" in host:
+                hosts = client.cmd(
+                    host, "test.true", expr_form="compound").keys()
+            else:
+                hosts = client.cmd(host, "test.true").keys()
+            if not hosts:
+                raise RuntimeError("No host matching '%s'" % (host,))
+            else:
+                return hosts
+        else:
+            return super(SaltBackend, cls).get_hosts(host, **kwargs)