fix(dao) allow self-signed certificates in migrations command

* postgres: allow self-signed certificates in Kong CLI
* cassandra: allow self-signed certificates in Kong CLI

This fixes a reported issue that Kong would not be able to run
migrations on PostgreSQL with self-signed certificates. The error "self
signed certificate" that Kong raised indicated that the root CA was not
made available to the cosocket in use.

Because Kong's CLI interpreter is resty-cli, it is too late to set the
resty-cli `lua_ssl_trusted_certificate` directive.

The approach we historically take is to rely on LuaSocket/LuaSec in
Kong's CLI and circumvent this limitation (the root CA file can be
specified at runtime as part of the LuaSocket instantiation options).

The Cassandra strategy has a similar fix following a slightly different
approach due to different underlying database drivers.

Fix #2856
From #2908

kong/dao/db/cassandra.lua

@@ -62,6 +62,12 @@ function _M.new(kong_config)
   if ngx.IS_CLI then
     local policy = require("resty.cassandra.policies.reconnection.const")
     cluster_options.reconn_policy = policy.new(100)
+
+    -- Force LuaSocket usage in order to allow for self-signed certificates
+    -- to be trusted (via opts.cafile) in the resty-cli interpreter.
+    -- As usual, LuaSocket is also forced in non-supported cosocket contexts.
+    local socket = require "cassandra.socket"
+    socket.force_luasocket("timer", true)
   end
 
   --

kong/dao/db/postgres.lua

@@ -62,8 +62,16 @@ end
 
 local function query_opts(self)
   local opts = self:clone_query_options()
-  opts.socket_type = forced_luasocket_phases[get_phase()] and
-                     "luasocket" or "nginx"
+
+  if ngx.IS_CLI or forced_luasocket_phases[get_phase()] then
+    -- Force LuaSocket usage in order to allow for self-signed certificates
+    -- to be trusted (via opts.cafile) in the resty-cli interpreter.
+    -- As usual, LuaSocket is also forced in non-supported cosocket contexts.
+    opts.socket_type = "luasocket"
+
+  else
+    opts.socket_type = "nginx"
+  end
 
   return opts
 end