controller: Fix usage of LabelSelector

[indradhanush]

sts.Spec.Selector is a map[string]string.

This leads to the following error on deleting pods:

ts=2018-07-10T16:05:30.055388286Z level=error component=controller/v1beta2 msg="Habitat could not be synced, requeueing" err="unable to parse requirement: invalid label key \"&LabelSelector{MatchLabels:map[string]string{habitat-name:\": name part must consist of alphanumeric characters, '-', '_' or '.', and must start and end with an alphanumeric character (e.g. 'MyName',  or 'my.name',  or '123-abc', regex used for validation is '([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9]')" obj=redis-habitat/redis

This PR fixes it.

[krnowak]

LFAD if tests pass.