Add test with a valid AC, containing a generic attribute

Add support for: user, user ca, server, server ca, server uri,
vo name, not before, not after, serial number, fqans.
Support for generic attributes requires a discussion on the
representation.

Resolve "Setup test proxy certificates and basic VOMS module testing"

See merge request storm2/ngx_http_voms_module!1

Merge branch '1-setup-nginx-voms-module-testing/1' of baltig.infn.it:storm2/ngx_http_voms_module into 1-setup-nginx-voms-module-testing/1

* '1-setup-nginx-voms-module-testing/1' of baltig.infn.it:storm2/ngx_http_voms_module:
  added line to run prove to README.md
  fixed README.md, adding links
  use relative paths for X509_VOMS_DIR and X509_CERT_DIR

Declare vomsdata globally and initialize it lazily at first use,
reusing the same object for all the Retrieve() calls.