check task groups for the update stanza if not defined at the job level

[fuleow]

We have job specs with the update stanza defined only at the task group but levant only checks at the job level. Resolves #145

This PR modified the checking logic so that If an update stanza is not defined at the job level it will check all task groups and return true if they all contain an update stanza.

[hashicorp-cla]

All committers have signed the CLA.