Envsetup: Add way to skip installing completions

Skip installing completion if ENVSETUP_NO_COMPLETIONS is set to 1.

Test: Following command with debug log added:
$ ENVSETUP_NO_COMPLETION=adb:fastboot:bit . build/envsetup.sh -> no completion installed
$ ENVSETUP_NO_COMPLETION=adb . build/envsetup.sh -> adb completion isn't installed
$ ENVSETUP_NO_COMPLETION=a . build/envsetup.sh -> all completions installed
$ ENVSETUP_NO_COMPLETION=fastboot . build/envsetup.sh -> fastboot completion isn't installed

Change-Id: I9f907585a1d2413f55ffcb6c7c01e49d1b55a3e0