Add diffie-hellman-group14-sha1 to the list of default key exchange algorithms

Add diffie-hellman-group14-sha1 to the list of default key exchange algorithms

The list of key exchange algorithms enabled by default has been changed
with a Mina SSHD uplift to the version of 2.6.0, leaving some of the
SHA1 algorithms disabled by default. Some the devices might still use
this algorithm, so this change brings back the
'diffie-hellman-group14-sha1' algorithm to the list.

JIRA: NETCONF-765
Change-Id: Iea58448981e19f1632799d05dbda8784415ebf1f
Signed-off-by: Oleksii Mozghovyi <oleksii.mozghovyi@pantheon.tech>