Add openstreetmap as basemap.

[na9da]

What this PR does

Adds openstreetmap as a basemap.
Sticking with current setup, this would still require change to simple.json in terriamap to use as default basemap.

Test me

• Open ci for this branch - http://ci.terria.io/add-openstreetmap
• From map settings select openstreet map as basemap

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[nf-s]

Awesome, thanks!

Just one small thing, can you please make the thumbnail a square so it matches the others

Thanks!

[na9da]

Awesome, thanks!

Just one small thing, can you please make the thumbnail a square so it matches the others

Thanks!

Yep, also I need to reduce the size of thumbnail.