Description of problem:
Verification of shared storage configuration values is missing.
Customer can provide any alphanumeric values as an input for shared storage configuration values with no verification. For example "gateway" key value like "nfs3" is accepted.

# hosted-engine --set-shared-config gateway nfs3 --type he_conf
# hosted-engine --get-shared-config gateway --type he_conf
gateway : nfs3, type : he_conf


Version-Release number of selected component (if applicable):
ovirt-hosted-engine-ha-2.1.5-1.el7ev.noarch
ovirt-hosted-engine-setup-2.1.3.6-1.el7ev.noarch

How reproducible:
100%

Steps to Reproduce:
1.Try setting and getting any improper key value to any possible he_conf key value, e.g. hosted-engine --set-shared-config port nfs3 --type he_conf.

Actual results:
There is no input verification exists for he_conf input key values.
# hosted-engine --set-shared-config port nfs3 --type he_conf
# hosted-engine --get-shared-config port --type he_conf
port : nfs3, type : he_conf


Expected results:
Each key value should be verified prior to being accepted and if unacceptable value provided, warning message should be provided for customer. 

Additional info:
Please see this pastebin for more technical details http://pastebin.test.redhat.com/509814.