MXS-1464 Add config 'substitute_variables'

With this variables set to true, if $VAR is used as a value in the
configuration file, then `$VAR` will be replaced with the value of
the environment variable VAR.

include/maxscale/config.h

@@ -157,6 +157,7 @@ extern const char CN_SSL_CERT_VERIFY_DEPTH[];
 extern const char CN_SSL_KEY[];
 extern const char CN_SSL_VERSION[];
 extern const char CN_STRIP_DB_ESC[];
+extern const char CN_SUBSTITUTE_VARIABLES[];
 extern const char CN_THREADS[];
 extern const char CN_THREAD_STACK_SIZE[];
 extern const char CN_TYPE[];
@@ -224,6 +225,7 @@ typedef struct
     char          admin_ssl_ca_cert[PATH_MAX];         /**< Admin SSL CA cert */
     int           query_retries;                       /**< Number of times a interrupted query is retried */
     time_t        query_retry_timeout;                 /**< Timeout for query retries */
+    bool          substitute_variables;                /**< Should environment variables be substituted */
 } MXS_CONFIG;
 
 /**