Add required_if to AnsibleModule

There is a common pattern in modules where some parameters are required
only if another parameter is present AND set to a particular value. For
instance, if a cloud server state is "present" it's important to
indicate the image to be used, but if it's "absent", the image that was
used to launch it is not necessary. Provide a check that takes as an
input a list of 3-element tuples containing parameter to depend on, the
value it should be set to, and a list of parameters which are required
if the required parameter is set to the required value.