security: use better PFS priority string

This patch changes the TLS priority string to remove RSA key-exchange
algorithms from current cipher suites (based on SECURE192).

This let DH key-exchange the only remaining algorithms.
Notice that we use a minimal of 2048-bits DH params
by using GNUTLS_SEC_PARAM_NORMAL settings
in the gnutls_sec_param_to_pk_bits function.

Using this priority makes audio and video calls forward secure,
a significant security and privacy feature for Ring users.

Tuleap: #383
Change-Id: Iabf51bb52617354c34410d15ee96e0be90acee55