Fix caching of Google auth tokens
Fix caching of Google auth tokens

_write_token_to_file was not zeroing the file before writing
a new token, causing corruption.

FIXES: LIBCLOUD-835

Closes #844

Signed-off-by: Tomaz Muraus <bb4af54d4692022d34bcf290dffd91d21e083d18@tomaz.me>

mgogoulos/libcloud,watermelo/libcloud,samuelchong/libcloud,erjohnso/libcloud,illfelder/libcloud,watermelo/libcloud,mistio/libcloud,Kami/libcloud,DimensionDataCBUSydney/libcloud,ByteInternet/libcloud,techhat/libcloud,illfelder/libcloud,erjohnso/libcloud,DimensionDataCBUSydney/libcloud,Kami/libcloud,andrewsomething/libcloud,NexusIS/libcloud,vongazman/libcloud,mgogoulos/libcloud,ZuluPro/libcloud,Kami/libcloud,NexusIS/libcloud,SecurityCompass/libcloud,ByteInternet/libcloud,t-tran/libcloud,NexusIS/libcloud,vongazman/libcloud,ZuluPro/libcloud,t-tran/libcloud,apache/libcloud,mistio/libcloud,t-tran/libcloud,techhat/libcloud,Scalr/libcloud,mistio/libcloud,samuelchong/libcloud,apache/libcloud,DimensionDataCBUSydney/libcloud,ByteInternet/libcloud,StackPointCloud/libcloud,watermelo/libcloud,erjohnso/libcloud,StackPointCloud/libcloud,andrewsomething/libcloud,andrewsomething/libcloud,SecurityCompass/libcloud,apache/libcloud,illfelder/libcloud,Scalr/libcloud,pquentin/libcloud,iPlantCollaborativeOpenSource/libcloud,pquentin/libcloud,iPlantCollaborativeOpenSource/libcloud,samuelchong/libcloud,techhat/libcloud,SecurityCompass/libcloud,ZuluPro/libcloud,mgogoulos/libcloud,StackPointCloud/libcloud,Scalr/libcloud,iPlantCollaborativeOpenSource/libcloud,vongazman/libcloud,pquentin/libcloud