Set certificate duration for external certificates to 100 years

We are going to allow user to choose the validity duration of the external certificate for cert manager. Right now it is 90 days, which might be suboptimal for some deployments. 

Community PR (https://github.com/percona/percona-server-mongodb-operator/pull/779) is great, but it sets default duration to 100 years, which is not aligned with security best practices. 

We are going to add a new section

spec:
  tls:

Under it we are going to have 

spec:
  tls:
    certValidityDuration: 

• Measured in days.
• 90 days is default