minion.conf: Accept tcp_keepalive* properties

Add TCP keepalive configuration options [1] to the list of accepted
minion config keys.

[1] https://docs.saltstack.com/en/latest/ref/configuration/minion.html

Change-Id: If80083fc793b86147b3691216dea571ea2cf0858
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

salt/files/minion.conf

   'max_event_size',
   'random_reauth_delay',
   'recon_default',
-  'recon_max'
+  'recon_max',
+  'tcp_keepalive',
+  'tcp_keepalive_cnt',
+  'tcp_keepalive_idle',
+  'tcp_keepalive_intvl'
 ] %}
 {%- for opt in opt_list %}
 {%- if minion.get(opt) %}