all: use buildlet client interface to enable testing

This change introduces the use of a buildlet client interface in
existing functions. This is being introduced in order to facilitate
testing.

Change-Id: I41cd5a372fc31edbd9bcba1859cdf84308360174
Reviewed-on: https://go-review.googlesource.com/c/build/+/371818
Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>
diff --git a/buildlet/ec2.go b/buildlet/ec2.go
index b6e6b45..c1a094d 100644
--- a/buildlet/ec2.go
+++ b/buildlet/ec2.go
@@ -39,7 +39,7 @@
 
 // StartNewVM boots a new VM on EC2, waits until the client is accepting connections
 // on the configured port and returns a buildlet client configured communicate with it.
-func (c *EC2Client) StartNewVM(ctx context.Context, buildEnv *buildenv.Environment, hconf *dashboard.HostConfig, vmName, hostType string, opts *VMOpts) (*Client, error) {
+func (c *EC2Client) StartNewVM(ctx context.Context, buildEnv *buildenv.Environment, hconf *dashboard.HostConfig, vmName, hostType string, opts *VMOpts) (Client, error) {
 	// check required params
 	if opts == nil || opts.TLS.IsZero() {
 		return nil, errors.New("TLS keypair is not set")