Run launch_on_reboot in launch_slaves

Every time we launch slaves on a given machine, we ensure that
launch-on-reboot is set up.

BUG=skia:2102

Review URL: https://codereview.chromium.org/675743003
Bring up new housekeeping slave on skia-vm-015

BUG=skia:3039

Review URL: https://codereview.chromium.org/640063007
Try again to centralize common initialization, this time named common.Init().

package init / func init() was calling too early,
package init / func Init() sadly is not legal go code (cannot import package as init - init must be a func) so
package common / func Init() will have to do.

BUG=skia:

Review URL: https://codereview.chromium.org/675843002
Special case for Android merge GCE instance

BUG=skia:

Review URL: https://codereview.chromium.org/671503007