Force go networking instead of cgo for cross and static builds

[riyazdf]

Identical fix to #1038 if we wanted to include it in a v0.4.3 release. Set up on a hotfix/0.4.3 branch

Signed-off-by: Riyaz Faizullabhoy riyaz.faizullabhoy@docker.com

[riyazdf]

CircleCI will fail only on container 4 during the $ buildscripts/circle_parallelism.sh (4) due to golang lint updates -- I'd prefer not to keep them in this PR to minimize code changes for this hotfix.

The rest of the tests pass, and here's an example output from make cross's linux client to show the fix works:

# ./notary version
notary
 Version:    0.4.3
 Git commit: 485d5e1
# ./notary -D list test-repo
DEBU[0000] Configuration file not found, using defaults
DEBU[0000] Using the following trust directory: /root/.notary
ERRO[0000] could not reach https://notary-server:4443: Get https://notary-server:4443/v2/: dial tcp: lookup notary-server on 8.8.8.8:53: no such host
INFO[0000] continuing in offline mode
DEBU[0000] No yubikey found, using alternative key storage: no library found

* fatal: client is offline

[endophage]

LGTM

[cyli]

LGTM