self-cert notary server with port is not support by docker trust in Docker for Windows

Description

Docker trust for notary server url with port (eg https://notary.example.com:4443 ) is not work in Docker for Windows

Docker will read certs from C:\Users\xxx.docker\tls\notary.example.com:4443 dirs, but I can not create dir with : in Windows.

I think certificateDirectory should be cleaned in Windows.

Steps to reproduce the issue:

1. create C:\Users\xxx.docker\tls\notary.example.com4443 dir and copy ca.crt to it
2. $env:DOCKER_CONTENT_TRUST_SERVER='https://notary.example.com:4443'
3. $env: DOCKER_CONTENT_TRUST=1
4. docker -D push index.example.com/foo/dotnet-samples:dotnetapp

Describe the results you received:
The push refers to repository [index.example.com/foo/dotnet-samples:dotnetapp]
xxxxxxx: Layer already exists
xxxxxxx: Layer already exists
xxxxxxx: Layer already exists
dotnetapp: digest: sha256:xxxxxx size: 2406
Signing and pushing trust metadata
DEBU[0001] reading certificate directory: C:\Users\xxx.docker\tls\notary.example.com:4443
DEBU[0001] Error pinging notary server "hosts://notary.example.com:4443/v2/": Get https://notary.example.com:4443/v2/: x509: certificate signed by unknow authority
DEBU[0001] Making dir path: C:\Users\xxx.docker\trust\tuf\index.example.com\foo\dotnet-samples\changelist
Error: error contacting notary server: x509: certificate signed by unknow authority

Describe the results you expected:
Push the image to registry and successfully signed

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client: Docker Engine - Community
 Version: 18.09.0
 API version:  1.39

Output of docker info:

(paste your output here)

Additional environment details (AWS, VirtualBox, physical, etc.):

No source file or test is named in the issue. Reproduce the Docker for Windows push with a notary URL containing port 4443, then trace how Docker CLI builds the certificate directory path from that URL. Done means the port-bearing notary server can use its CA certificate and the trust metadata push succeeds.