Optimizing number of calls made to sonic-cfggen during service
start up as it adds to total system boot up time.
***-Test 1***
there is an average saving of 1 to 1.5 sec between old script and new script
```
root@str-s6000-acs-14:/# time /usr/bin/rest-server-old.sh
Generating temporary TLS server certificate ...
2020/07/09 19:03:33 wrote cert.pem
2020/07/09 19:03:33 wrote key.pem
REST_SERVER_ARGS = -ui /rest_ui -logtostderr -cert /tmp/cert.pem -key /tmp/key.pem
/usr/sbin/rest_server -ui /rest_ui -logtostderr -cert /tmp/cert.pem -key /tmp/key.pem
real 0m8.790s
user 0m7.993s
sys 0m0.584s
root@str-s6000-acs-14:/# time /usr/bin/rest-server-new.sh
Generating temporary TLS server certificate ...
2020/07/09 19:03:45 wrote cert.pem
2020/07/09 19:03:45 wrote key.pem
REST_SERVER_ARGS = -ui /rest_ui -logtostderr -cert /tmp/cert.pem -key /tmp/key.pem
/usr/sbin/rest_server -ui /rest_ui -logtostderr -cert /tmp/cert.pem -key /tmp/key.pem
real 0m6.940s
user 0m5.670s
sys 0m0.386s
```
***-Test 2***
Built an image with this change and rest server is running with params as described in test 1 above
```
admin@str-s6000-acs-14:~$ ps -ef | grep rest_server
root 3301 2866 2 02:09 pts/0 00:00:10 /usr/sbin/rest_server -ui /rest_ui -logtostderr -cert /tmp/cert.pem -key /tmp/key.pem
```
signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>
