Improve nginx TLS cipher list & session timeout

This disables the cipher RC4-MD5, enables ciphers (especially for forward
secrecy) and makes the default session timeout explicit.
Loosely based on https://wiki.mozilla.org/Security/Server_Side_TLS#Nginx
except that DHE and other CPU-expensive ciphers are not enabled.

In tests, this has shown some increase in server load and a 10-15 %
decrease in throughput (depending on browser), but nothing worrisome.

Bug: 53259
Change-Id: I830bec497338a36f99ae90536961180dab8b5612