Update jira to only add watchers that are missing

[qyeo-sc]

There are two issues that this PR fixes:

1. When self.client.add_watcher(issue.jira_issue, watcher) fails previously, it'd throw an exception resulting in the entire .save or .create call to fail, which would retry continuously hammering the jira instance.
2. The function would always try to add watchers to issues regardless if the watchers were already added previously.

[qyeo-sc]

@jonathanmetzman would you be able to take a look whenever you have the time?

[qyeo-sc]

Is there any unit tests that can be added ?

@javanlacerda added some unit tests here

[qyeo-sc]

@javanlacerda could you take another look whenever you have the chance?