Rename Mainflux to Magistrala in HELM charts

[JeffMboya]

1. Is there an open issue addressing this request? If it does, please add a "+1" reaction to the existing issue, otherwise proceed to step 2.

No

2. Describe the enhancement you are requesting, as well as the possible use case(s) for it.

Rename "Mainflux" to "Magistrala" in HELM charts. This will make the naming more consistent.

3. Indicate the importance of this feature to you (must-have, should-have, nice-to-have).

Must-have.