MULE-7023: Rename keep-alive attribute in HTTP/HTTPS endpoints

A new keepAlive attribute was created in the schema. Tests for HTTP inbound/outbound endpoints were changed to use this property. Inbound endpoint tests which use services still use the old keep-alive property (in order to check that it still works).
Merge pull request #58 from mulesoft/MULE-7023

MULE-7023: Rename keep-alive attribute in HTTP/HTTPS endpoints