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/backends.py

@@ -27,6 +27,7 @@ from moto.s3 import s3_backends
 from moto.ses import ses_backends
 from moto.sns import sns_backends
 from moto.sqs import sqs_backends
+from moto.ssm import ssm_backends
 from moto.sts import sts_backends
 
 BACKENDS = {
@@ -56,6 +57,7 @@ BACKENDS = {
     'ses': ses_backends,
     'sns': sns_backends,
     'sqs': sqs_backends,
+    'ssm': ssm_backends,
     'sts': sts_backends,
     'route53': route53_backends,
     'lambda': lambda_backends,