How to log into git bitbucket repository from jenkins
Specifying an IdentityFile in your config doesn't change whether you have to provide a password for the ssh key. That'd be a gaping security hole if it did. The only way you can not be required to provide a password for your ssh key is if your ssh key doesn't have a password.
So, you should create an ssh key specifically for your jenkins instance and, if Bitbucket allows, only give read-only access to your repository for that key.
This might be related to https://issues.jenkins-ci.org/browse/JENKINS-20879Using a key without passphrase works for me.