azure: bootstrapping prints out scary, spurious ERROR messages

[Andrew Wilkins]

$ juju bootstrap --upload-tools azure azure
Creating Juju controller "local.azure" on azure/southeastasia
ERROR found unknown VM size "Standard_D15_v2"
ERROR found unknown VM size "Standard_DS1_v2"
ERROR found unknown VM size "Standard_DS2_v2"
ERROR found unknown VM size "Standard_DS3_v2"
ERROR found unknown VM size "Standard_DS4_v2"
ERROR found unknown VM size "Standard_DS5_v2"
ERROR found unknown VM size "Standard_DS11_v2"
ERROR found unknown VM size "Standard_DS12_v2"
ERROR found unknown VM size "Standard_DS13_v2"
ERROR found unknown VM size "Standard_DS14_v2"
ERROR found unknown VM size "Standard_DS15_v2"
...

Those ERRORs used to be WARNINGs, until a recent (invalid) change. We still enable the user to select unknown (to the Juju client) VM sizes, but we just consider them of equal cost, but more expensive than anything the client *does* know about.

[Andrew Wilkins (axwalk)]

I'll change the logging level to DEBUG. They don't need to be shown to the user by default.