Use user.signingkey for tag-signing.
Use user.signingkey for tag-signing.

Rather than use UID require an explicitly-set key ID, following recommendations at [1].

[1] http://www.apache.org/dev/openpgp.html

Testing Done:
Tested with user.signingkey set and unset on local merged master.

Reviewed at https://reviews.apache.org/r/18245/

mkhutornenko/incubator-aurora,shahankhatch/aurora,medallia/aurora,rdelval/aurora,rdelval/aurora,mkhutornenko/incubator-aurora,shahankhatch/aurora,protochron/aurora,apache/aurora,mschenck/aurora,mkhutornenko/incubator-aurora,mschenck/aurora,rdelval/aurora,rdelval/aurora,kidaa/aurora,crashlytics/aurora,shahankhatch/aurora,medallia/aurora,crashlytics/aurora,medallia/aurora,wfarner/aurora,thinker0/aurora,crashlytics/aurora,wfarner/aurora,medallia/aurora,protochron/aurora,apache/aurora,kidaa/aurora,mschenck/aurora,mkhutornenko/incubator-aurora,thinker0/aurora,apache/aurora,thinker0/aurora,rosmo/aurora,apache/aurora,shahankhatch/aurora,wfarner/aurora,rosmo/aurora,rdelval/aurora,rosmo/aurora,mschenck/aurora,thinker0/aurora,rosmo/aurora,mschenck/aurora,kidaa/aurora,crashlytics/aurora,protochron/aurora,rosmo/aurora,kidaa/aurora,wfarner/aurora,protochron/aurora,medallia/aurora,kidaa/aurora,rdelval/aurora,apache/aurora,protochron/aurora,thinker0/aurora,apache/aurora,thinker0/aurora,shahankhatch/aurora,wfarner/aurora,medallia/aurora,crashlytics/aurora,crashlytics/aurora,kidaa/aurora,mschenck/aurora,protochron/aurora,mkhutornenko/incubator-aurora,shahankhatch/aurora,rosmo/aurora,wfarner/aurora