[Docs] Add note for Fuel Repository Mirroring

https://docs.mirantis.com/openstack/fuel/fuel-7.0/reference-architecture.html#fuel-repository-mirroring

After fuel-createmirror command default repositories into Fuel will be changed to local
Please add warning into documentation about it

How to reproduce:

1 Install Fuel 7.0
2 create env
3 Follow to Settings -> Repositories, change some repository or delete it.
4 press "Load defaults" -> all repositories will be restored

# fuel --env 3 settings -d

      type: custom_repo_configuration
      value:
      - name: ubuntu
        priority: null
        section: main universe multiverse
        suite: trusty
        type: deb
        uri: http://archive.ubuntu.com/ubuntu/
      - name: ubuntu-updates
        priority: null
        section: main universe multiverse
        suite: trusty-updates
        type: deb
        uri: http://archive.ubuntu.com/ubuntu/
      - name: ubuntu-security
        priority: null
        section: main universe multiverse
        suite: trusty-security
        type: deb
        uri: http://archive.ubuntu.com/ubuntu/
      - name: mos
        priority: 1050
        section: main restricted
        suite: mos7.0
        type: deb
        uri: http://10.200.0.2:8080/2015.1.0-7.0/ubuntu/x86_64
      - name: Auxiliary
        priority: 1150
        section: main restricted
        suite: auxiliary
        type: deb
        uri: http://10.200.0.2:8080/2015.1.0-7.0/ubuntu/auxiliary

5 run fuel-createmirror on fuel-master node
6 After fuel-createmirror ends folloe to the Fuel WEB UI, Settings -> Repositories, press "Load defaults" -> all repositories will be restored to the NEW default repositories

      extra_priority: null
      type: custom_repo_configuration
      value:
      - name: trusty
        priority: null
        section: main
        suite: trusty
        type: deb
        uri: http://10.200.0.2:8080/ubuntu-part
      - name: mos
        priority: 1050
        section: main restricted
        suite: mos7.0
        type: deb
        uri: http://10.200.0.2:8080/2015.1.0-7.0/ubuntu/x86_64
      - name: mos-updates
        priority: 1050
        section: main restricted
        suite: mos7.0-updates
        type: deb
        uri: http://10.200.0.2:8080/mos-ubuntu
      - name: mos-security
        priority: 1050
        section: main restricted
        suite: mos7.0-security
        type: deb
        uri: http://10.200.0.2:8080/mos-ubuntu
      - name: mos-holdback
        priority: 1100
        section: main restricted
        suite: mos7.0-holdback
        type: deb
        uri: http://10.200.0.2:8080/mos-ubuntu
      - name: Auxiliary
        priority: 1150
        section: main restricted
        suite: auxiliary
        type: deb
        uri: http://10.200.0.2:8080/2015.1.0-7.0/ubuntu/auxiliary

and we do not have any way to restore real default repositories pointed to upstream, only manual.