bnc#829301 - Unsigned repo warning

bnc#829301 - Installing packages from Cloud-PTF gives unsigned repo warning
This commit creates a gpg key on the admin node, if it does not already exist,
and signs the SUSE-Cloud-PTF repository and copies the public key to the
repository
Added changes as requested in the piull-request discussion
Call create_gpg_key from sign_repositories, not the opposite

Also change some strings again :-)
Merge pull request #1 from vuntz/bnc829301

Call create_gpg_key from sign_repositories, not the opposite
Only sign Cloud-PTF repository if it exists
Merge pull request #2 from vuntz/bnc829301

Only sign Cloud-PTF repository if it exists
Merge pull request #1889 from nkrinner/bnc829301

bnc#829301 - Unsigned repo warning