Change VOMS warning message when requesting a too long proxy

[federicaagostini]

Right know, when a client requests a too long proxy it gets a warning like The validity period of the issued attributes has been shortened to the maximum allowed by this VOMS server configuration.

We can improve this message also to be coherent with the legacy VOMS warning, as @giacomini suggests:

The validity of this VOMS AC in your proxy is shortened to <seconds> seconds, which is the maximum allowed by this VOMS server configuration.

After the fix, the following tests should be green

• See if requesting a too long proxy fails
• See if requesting a too long ac length fails.

[federicaagostini]

PR https://github.com/indigo-iam/iam/pull/756