[yugabyted] yugabyted doesn't verify join flag when passed through config file.

[gargsans-yb]

Jira Link: DB-11929

Description

When starting a cluster, if the join flag is provided through the config file, yugabyted doesn't complete the config validation step for join flag causing the node to start as a read-replica node.

Steps to replicate:

Start the 1st node: 
./yugabyted start --advertise_address 127.0.0.1 --base_dir ~/yb-cluster/node1

Create a `ybd.conf` file with following content:

{
     "join": "127.0.0.1"
}


Start the 2nd node: 
./yugabyted start --advertise_address 127.0.0.2 --base_dir ~/yb-cluster/node2 --config=~/ybd.conf

This causes the 2nd node to start as a read replica as the placement UUID set for both nodes are different.

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