Changed ros_datacentre in strands-base.yaml to mongodb_store

for the next system update please do the following:
`cd /opt/strands/strands_catkin_ws/src`
`wstool rm ros_datacentre`
`rm -rf ros_datacentre`
`wstool merge https://raw.githubusercontent.com/strands-project/strands_systems/hydro-devel/strands_rosinstall/strands-base.yaml`
`wstool update`
Merge pull request #73 from cdondrup/hydro-devel

Changed ros_datacentre in strands-base.yaml to mongodb_store