Validate SP path settings during installation

Validate SP path settings during installation

There are a number of URL path options that can be specified as
options when running ipsilon-client-install. There are certain
rules that must be followed to result in a valid mod_auth_mellon
configuration:

 - All path options must be prefixed with '/'.

 - The mellon endpoint path (--saml-sp) must be a subpath of the
   httpd 'Location' element is it contained within (--saml-base).

 - The logout (--saml-sp-logout) and post (--saml-sp-post) paths
   must be subpaths of the mellon endpoint (--saml-sp).

This adds validation for all of the above rules.

https://fedorahosted.org/ipsilon/ticket/82

Signed-off-by: Nathan Kinder <nkinder@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>