Bug: "--batch_size_percentage" option doesn't work

[v-stepanov]

Example call:

senza respawn-instances nakadi-archiver-staging default --region=eu-central-1 --batch_size_percentage=100

Error message:

File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/senza/cli.py", line 1956, in respawn_instances
    group["resource_id"], group["stack_name"], region, batch_size_percentage
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/senza/respawn.py", line 186, in respawn_elastigroup
    if batch_size is None or batch_size < 1:
TypeError: '<' not supported between instances of 'str' and 'int'