[qos reload] Fix "config qos reload" overriding entire CONFIG_DB

[stepanblyschak]

What I did

config qos reload command uses a combination of sonic-cfggen's flags -d and --write-to-db that makes it override entire CONFIG_DB, updating every key. This leads to issues with Orchs, daemons that do not support updating keys in CONFIG_DB. Best case, it causes errors in logs.

How I did it

First, render templates to temporary files, then load those files into CONFIG_DB.
Also, fixed an issue where using dry_run option only produced QOS config but not the buffer configuration and updated test files accordingly.

How to verify it

Run on switch:

root@r-bulldog-04:/home/admin# config qos reload
Running command: /usr/local/bin/sonic-cfggen -d -t /usr/share/sonic/device/x86_64-mlnx_msn2100-r0/ACS-MSN2100/buffers_dynamic.json.j2,/tmp/cfg_buffer.json -t /usr/share/sonic/device/x86_64-mlnx_msn2100-r0/ACS-MSN2100/qos.json.j2,/tmp/cfg_qos.json -y /etc/sonic/sonic_version.yml
Running command: /usr/local/bin/sonic-cfggen -j /tmp/cfg_buffer.json -j /tmp/cfg_qos.json --write-to-db

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)