Permit settings check for sp only

[toopy]

I need to validate SP settings only to expose my metadata and IdP + SP if I work on ACS validation on IdP-Initiated POST:

• https://github.com/onelogin/python-saml/blob/master/src/onelogin/saml2/settings.py#L306

What do you think about splitting the check_settings method in OneLogin_Saml2_Settings in two new ones check_sp_settings and check_idp_settings (called in the current one). Could you be interested for a PR in this way ?

[pitbulk]

I need to review implications, but PRs are always welcome.