Fix mac checker regex
Fix mac checker regex

Currently, the mac checker regex could match a corner case of
11:22:33:44:55:66: (one extra colon at the end). We fix this, and we
also move the regex compilation outside of this function, at module
level.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

grnet/snf-ganeti,dimara/ganeti,andir/ganeti,andir/ganeti,grnet/snf-ganeti,andir/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,apyrgio/ganeti,leshchevds/ganeti,bitemyapp/ganeti,leshchevds/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,yiannist/ganeti,mbakke/ganeti,leshchevds/ganeti,ganeti/ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti,bitemyapp/ganeti,yiannist/ganeti,mbakke/ganeti,yiannist/ganeti,onponomarev/ganeti