Added a simple support of global Perforce credentials

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Fixes

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Use default credentials in validation forms (if the job-specific user is missing)

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #57 from synopsys-arc-oss/custom-global-credentials

Added a simple support of global Perforce credentials