Fix Traffic Ops looking for influxdb.conf in the wrong place

[ocket8888]

What does this PR (Pull Request) do?

• This PR fixes Traffic Ops Golang isn't looking for influxdb.conf in the right place #4292

With this PR, Traffic Ops will look for influxdb.conf in ./conf/$MOJO_MODE/ if influxdb_conf_path is not given in cdn.conf and the MOJO_MODE environment variable is set to any non-empty string.

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Set MOJO_MODE, then start Traffic Ops from somewhere such that there is a valid influxdb.conf at ./conf/$MOJO_MODE/influxdb.conf, then verify that the therein contained was picked up by Traffic Ops by requesting some endpoint that needs Traffic Stats and watching the logs output.

If this is a bug fix, what versions of Traffic Control are affected?

• 4.1.0
• master

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY

[rawlinp]

LGTM