[2.2] Update coordination strategy of FlatSearchCommandHandler

[Skoucail]

Add MRCluster_MastersOnly to the coordination strategy of the FlatSearchCommandHandler
This fixes #326

[Skoucail]

@rafie
Can you help me?
I have no clue why that 1 single test fails on the build server because of my PR

[rafie]

Hello @Skoucail, this is a flaky test, just needs a re-run.
However, please note we no longer accept PRs on the RSCoordinator repo (as RSCoordinator is now integrated into the RediSearch repo). Instead, please open a PR on RediSearch/master, and modify files under the coord directory. Thanks!

[Skoucail]

@rafie
Oh okee, didnt know this repo is depricated now.
I did a test with the RediSearch 2.2 branch. Same issue.
So i recreated the pull request over in RediSearch: RediSearch/RediSearch#2404

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}