tls: Fix conversion of dh_params::level to gnutls_sec_param_t
tls: Fix conversion of dh_params::level to gnutls_sec_param_t

The old code was casting level to gnutls_sec_param_t directly, but
numeric constants defined in level do not match constants defined in
gnutls.h, at least in my version (3.4.10-1.fc23.x86_64). For instance,
level::LEGACY is 2 while GNUTLS_SEC_PARAM_LEGACY is 30.

Fixes the following tls_test failures:

$ build/release/tests/tls_test
Running 6 test cases...
unknown location(0): fatal error in "test_simple_x509_client_server": std::runtime_error: An illegal parameter was found.
tests/test-utils.cc(32): last checkpoint
unknown location(0): fatal error in "test_simple_x509_client_server_again": std::runtime_error: An illegal parameter was found.
tests/test-utils.cc(32): last checkpoint
tests/tls_test.cc(211): fatal error in "test_x509_client_server_cert_validation_fail": Unexpected exception
tests/tls_test.cc(226): fatal error in "test_x509_client_server_cert_validation_fail_name": Unexpected exception

Message-Id: <ad702883251b76213221209feed8c20bd3e37a60@scylladb.com>

cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,kjniemi/seastar,kjniemi/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,avikivity/seastar,kjniemi/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar