Add a master switch so that the sync/async mode can be configured.
Example usage of the switch:
1. Configure mode while building an image
`make ENABLE_SYNCHRONOUS_MODE=y <target>`
2. Configure when the device is running
Change CONFIG_DB with `sonic-cfggen -a '{"DEVICE_METADATA":{"localhost": {"synchronous_mode": "enable"}}}' --write-to-db`
Restart swss with `systemctl restart swss`
Optimizing number of calls made to sonic-cfggen during service
start up as it adds to total system boot up time.
signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>
**- Why I did it**
sonic-cfggen call is slow and it adds to system start up time
**- How I did it**
places all required variable into single template and called into sonic-cfggen using this template
**- How to verify it**
***-Test 1***
there is an average saving of .5 to 1 sec between old script and new script
```
root@str-s6000-acs-14:/# time ./orchagent_old.sh
/usr/bin/orchagent -d /var/log/swss -b 8192 -m f4:8e:38:16:bc:8d
real 0m3.546s
user 0m2.365s
sys 0m0.585s
root@str-s6000-acs-14:/# time ./orchagent_new.sh
/usr/bin/orchagent -d /var/log/swss -b 8192 -m f4:8e:38:16:bc:8d
real 0m2.058s
user 0m1.650s
sys 0m0.363s
```
***-Test 2***
Built an image with this change and orchagent is running with intended params:
```
admin@str-s6000-acs-14:~$ ps -ef | grep orchagent
root 2988 1901 1 02:09 pts/0 00:00:02 /usr/bin/orchagent -d /var/log/swss -b 8192 -m f4:8e:38:16:bc:8d
```
signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>
