Add version of azure-mgmt-resource in requirements.txt

[LakshyAAAgrawal]

I get the following INFO on running pip install -r requirements.txt. It can be fixed by providing the version of azure-mgmt-resource used in the authors environments.

INFO: pip is looking at multiple versions of azure-mgmt-resource to determine which version is compatible with other requirements. This could take a while.

[zackhuiiiii]

thanks!