Parse decimals as password from environment variable overrides correctly.
Prior to this change, config values for passwords are not allowed to be decimals.
e.g. `EMQX_FOOBAR__PASSWORD=12344` or `emqx.foobar.password=1234`
would result in a type check error, unless quoted as:
`EMQX_FOOBAR__PASSWORD='"12344"'` or `emqx.foobar.password="1234"`.
After this fix, the value does not have to be quoted.