Implement support for SSM parameter store

This commit adds initial support for the Simple System Manager client.

It currently only mocks the following api endpoints:
 - delete_parameter()
 - put_parameter()
 - get_parameters()

moto/ssm/models.py

@@ -0,0 +1,65 @@
+from __future__ import unicode_literals
+
+from moto.core import BaseBackend, BaseModel
+from moto.ec2 import ec2_backends
+
+
+class Parameter(BaseModel):
+    def __init__(self, name, value, type, description, keyid):
+        self.name = name
+        self.type = type
+        self.description = description
+        self.keyid = keyid
+
+        if self.type == 'SecureString':
+            self.value = self.encrypt(value)
+        else:
+            self.value = value
+
+    def encrypt(self, value):
+        return 'kms:{}:'.format(self.keyid or 'default') + value
+
+    def decrypt(self, value):
+        if self.type != 'SecureString':
+            return value
+
+        prefix = 'kms:{}:'.format(self.keyid or 'default')
+        if value.startswith(prefix):
+            return value[len(prefix):]
+
+    def response_object(self, decrypt=False):
+        return {
+            'Name': self.name,
+            'Type': self.type,
+            'Value': self.decrypt(self.value) if decrypt else self.value
+        }
+
+
+class SimpleSystemManagerBackend(BaseBackend):
+
+    def __init__(self):
+        self._parameters = {}
+
+    def delete_parameter(self, name):
+        try:
+            del self._parameters[name]
+        except KeyError:
+            pass
+
+    def get_parameters(self, names, with_decryption):
+        result = []
+        for name in names:
+            if name in self._parameters:
+                result.append(self._parameters[name])
+        return result
+
+    def put_parameter(self, name, description, value, type, keyid, overwrite):
+        if not overwrite and name in self._parameters:
+            return
+        self._parameters[name] = Parameter(
+            name, value, type, description, keyid)
+
+
+ssm_backends = {}
+for region, ec2_backend in ec2_backends.items():
+    ssm_backends[region] = SimpleSystemManagerBackend()