What if I max out my contributions to my IRA and later in the year
(say in Aug 2017), I switch to an employer who does provide a 401k.
Will that make me pay a fine for the traditional IRA that I have
opened back when my employer did not provide me a 401k?
There is no income limit for contributing to a Traditional IRA. As long as you make at least $5500 that year, you can contribute $5500 to a Traditional IRA regardless of how high your income is and regardless of whether you contributed to a 401(k) that year. Therefore, you will not have an "excess contribution" that would trigger a penalty if you didn't withdraw it.
However, if you or your spouse contributed to a 401(k) that year, then there is an income limit on your ability to deduct a Traditional IRA contribution. So if you contribute (not just have one, but contribute, by either you or the employer) to the 401(k) from the new employer during the year, then you may suddenly find yourself unable to deduct the Traditional IRA contribution you made earlier when you file taxes for the year, which basically means it's a "non-deductible" Traditional IRA contribution.