[sensuctl] splay_coverage should not be mandatory

[amdprophet]

The splay_coverage attribute in proxy check requests should have a default of 90 as described by the documentation. It is currently a mandatory field so the default is never used.

➜  sensu-go (master) ✗ cat proxy_requests.json
{
    "entity_attributes": ["entity.Class == 'agent'"],
    "splay": true
}

➜  sensu-go (master) ✗ bin/sensuctl check set-proxy-requests ping -f proxy_requests.json
Error: proxy request splay coverage must be greater than 0 if splay is enabled