AwsEc2Cache Refresh() fix

[faja]

Currently Refresh() function creates new AwsEc2Provider each time it is
executed. Morover it sets AwsConfig field to an "empty" aws.Config{}
causing aws.Config.Region to be . This forces REGION to be taken
from ~/.aws/config profile instead of .goship.yaml config file.

It could be fixed by:
"AwsConfig: config" -> "AwsConfig: g.Provider.AwsConfig":

-       config := aws.Config{}
-       provider := providers.AwsEc2Provider{AwsConfig: config, AwsRegion: g.Provider.AwsRegion, AwsProfileName: g.Provider.AwsProfileName}
+       provider := providers.AwsEc2Provider{AwsConfig: g.Provider.AwsConfig, AwsRegion: g.Provider.AwsRegion, AwsProfileName: g.Provider.AwsProfileName}

or (imo) even better to simply reuse existing provider without creating
a new struct. And that's what I'm suggesting in this commit.

[emate]

thanks @faja!