Pools can now be each be assigned an OpenSSL cipher string with the
field tls_ciphers. A new configuration option, default_pool_ciphers,
specifies what cipher string to use for new tls-enabled pools
if one is not explicitly specified at time of creation.
Change-Id: Iedb7774bfb8d70ea307d6a513248e1fe2389fa34
Depends-On: I77da6f14063877af0077f2c12df1aab5d5ead187
Story: 2006627
Task: 37172