Error happened when s3's provider is not aws "Unable to connect to instance metadata service"

[blademainer]

When s3's api is not provided by AWS(say, googlecloud, azure), an error would happen. because the endpoint-url is overwrited by the region conditional judgment: https://github.com/hackmdio/codimd/blob/a41c4db0c207caf62e9216b32db194aa1cbefea4/lib/imageRouter/s3.js#L51-L57

my suggest code to fix this bug:

      let s3Endpoint = 's3.amazonaws.com'
      if (config.s3.region && config.s3.region !== 'us-east-1') {
        s3Endpoint = `s3-${config.s3.region}.amazonaws.com`
      }
      if (config.s3.endpoint) {
        s3Endpoint = config.s3.endpoint
      }