worker/uniter/jujuc: Fix TestAddMetric on gccgo

Change logic of add-metric command to avoid relying on map ordering
in tests, which is undefined and different on gccgo. Rather than
recording metrics as a map, keep it as a slice, but use a map when
parsing to check and error still on duplicate keys.

Also shortens error message on duplicate keys and makes output
from TestAddMetrics clearer.
Changed to ignore gccgo as build directive.
Merge pull request #897 from anastasiamac/Bug1378716

Changed to ignore gccgo as build directive.
Merge pull request #895 from bz2/test_add_metric_1379208

worker/uniter/jujuc: Fix TestAddMetric on gccgo

Change logic of add-metric command to avoid relying on map ordering
in tests, which is undefined and different on gccgo. Rather than
recording metrics as a map, keep it as a slice, but use a map when
parsing to check and error still on duplicate keys.

Also shortens error message on duplicate keys and makes output
from TestAddMetrics clearer.

http://reviews.vapour.ws/r/159/
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